With respect to getting too-lite loads in the 357 Mag case, the question was about how to get the same velocity in the two cases. If you don't go below published 38 Special loads in the 38 Special case, then getting the same velocity in the 357 Mag case will not result in a too-lite load, either. It will actually provide a little more powder, a little more gas, and a little more margin against sticking a bullet in the barrel.
With respect to the computerized load programs, that kind of tradeoff is easily calculated. But, those programs are not that accurate for revolvers.
Heck, even the manual data is not that accurate for predicting revolver velocities. The newest Speer manual has some comparison data for 3 loads in 26 different revolvers. Each revolver got identically loaded cartridges and velocities were measured the same way for each. Just looking at the nine revolvers with 6" barrels, there is a spread of 1237 to 1603 fps for a 110 gr bullet, 1142 to 1417 fps for a 140 gr bullet, and 1002 to 1284 fps for a 158 gr bullet. The only thing you can say about consistency is that the same gun was the fastest with all three loads, and another particular gun was always the slowest. Sooo, if you go to the load data for the 38 Special and see that the test gun was a S&W Model 14 with a 6" barrel, then go to the 357 Mag data and find that the test gun was a S&W Model 19 with a 6" barrel, you can bet that those two guns still won't shoot the same 38 Special cartridge load at the same velocity. So, how do you use THAT 2 sets of data to find the equivalent velocities for two loads for the same 38 Special case that will shoot at the same velocity from your ONE gun? You would need that info before you can very accurately decide how to adjust the load to get the same velocity in the 357 Mag case. But, the manual won't give you that information unless they happen to use the SAME TEST GUN for the 38 Special and 357 Mag data, AND they use the data format that shows several velocities with loads for each with several powders. NONE of my manuals fit that requirement! Anybody else have one that does?
So, all things considered, what I do to answer a qustion like this is a simple calcualtion to keep the ratio of powder charge weight to powder space the same (called the loading density). It is based on the case capacities measured in grains of water and some length measurements of the bullet and the loaded cartridges.
I'll illustrate the method with an example that starts with a max load for the 38 Special from the Accurate Arms 2nd edition manual:
Hornady 158 gr XTP with 4.0 gr of No.2 powder for 16,500 psi and 756 fps from aS&W K-38 with a 8-3/8" barrel.
Using my notes for water capacity measurement of 357 Mag brass fired in MY gun, I have a full-case capacity of 26.1 gr of water for a case that is 1.290" long. A Hornady XTP measured 0.668" long and gave a cartridge OAL of 1.590 when crimped at the cannelure. So, there is 1.290" + 0.668" - 1.590" = 0.388" of bullet inside the 357 Mag case. The volume of the part of the bullet that is inside the case is calculated by multiplying the diameter by itself (i.e., squaring the number) dividing that by 4 and multiplying it by pi (i.e., 3.1416) to get the area of the bullet's base. For a .357" bullet, that is: .357" x .357" / 4 x 3.1416 = 0.100 square inch (a very easy number to remember). So, the area of the base times the length in the case gives 0.100 x 0.388 = 0.0388 cubic inches. We need to subtract that volume from the full-case capacity, but the two volumes were not measured in the same units. We need to convert the cubic inches to an equivalent weight of water. To do that, multiply by 252.8 grains per cubic inch. So, the NET water capacity with the bullet seated is 26.1 - 252.8 x 0.0388 = 16.3 gr of water.
Now, we do the same thing with a 38 Special case. We really should measure the water capacity of the 38 Special casee, because they are not the same internally as a shortened 357 Mag case. But, I don't have that water capacity measurement in my notes, so I am going to cheat a little here and estimate the 38 Special case volume based on shortening the 357 Mag case by the difference in case lengths: 1.290" - 1.155" = 0.135". I'll calculate the volume lost to case shortening the same way I calculated volume lost to bullet seating: 0.100 sq inch x 0.135 inch x 252.8 grains per cubic inch = 3.4 grains of water. So, I now have a net water capacity of 16.3 - 3.4 = 12.9 grains for the 38 Special case.
The ratio of charge weight to NET water for the example 38 Special load is 4 gr powder to 12.9 gr water, or 4 /12.9 = 0.310 in the 38 Special case. To get the same loading density (ratio)in the larger 357 Mag case, I need to multiply the larger case capacity by this ratio: 16.3 x 0.310 = 5.053 gr of No.2 powder.
This should give about the same velocity for both cartridges in the same gun. Using a chronograph would allow further tuning the loads to exactly the same velocity.
I will also acknowledge that the new Speer manual does not list 158 gr jacketed bullet loads for the 38 Special and says the reason is that the velocities are too low so that there is some possibility of a bullet sticking in the barrel. This warning is a new trend. It raises questions about a lot of commercially available ammo as well as a lot of existing handloading data. Speer would not (now) provide a load for #2 powder with a 158 gr bullet in the 38 Special case, although Speer used to do so and most manuals still do. So, if you are using 4.0 grains of No.2 in the 38 Special with 158 grain jacketed bullets (or 5.1 grains in 357 cases) I guess you should be careful that you know each bullet has left your barrel before you pull your trigger again.
At this point, let me caution people who are not mathematically inclinded that they cannot apply the ratio from this example to loads for other bullets. Obvioulsy, bullets of different weights will take up more or less of the case capacity and thus have a different net powder space volume ratio. Even bullets of the same weight can have different seating depths. So, a large hollow point and a full-metal jacket bullet of the same weight will have different lengths, and even 2 JHPs from different manufacturers can have different seating depths due to different cannelure locations. Lead bullets with SWC noses and lube grooves can have much different seating depths than JHPs, and so-on. So, don't fool yourselves with shortcuts that aren't correct.
SL1