The more "normal" thing if you don't have one of the common long chambers, is to regulate seating depth such that the leading edge of the bullet bearing surface (the full diameter cylindrical portion) to be about 0.020" forward of the mouth of a maximum length (0.898") case. It will be longer when the case is shorter, and .45 Auto brass does tend to shorten around half a thousandth per load cycle, rather than lengthen as high pressure rounds do. As a result of different bullet shapes, the COL can be all over the map when meeting this criterion. The Official SAAMI spec is 1.190"-1.275", but that just applies to round nose. Enough odd cast bullet shapes have been made that this doesn't begin to cover everything that's available.
One thing you do need to watch out for is that pressure can rise significantly as you seat bullets deeper because you are asking the powder to start making gas in a tighter confinement. This is particularly true of short pistol case ammo because a small increase in seating depth represents a big percentage reduction of the total powder, where in a longer case or one with a bottleneck it represents a smaller percentage and it therefore less critical to pressure. Speer, I think it was, at one point had an example of a 9 mm going to over 60,000 psi because of deep seating. If you increase seating depth, back the load off and work it up again while watching for pressure signs.
What the above means is, for any particular powder, the deeper you seat the bullet, the less velocity you can get using that powder before you exceed pressure limits. Most folks will have observed that they get more velocity seating deeper, thinking they've got it for free, but actually the pressure has gone up. Fortunately, most standard loads have some wiggle room, and as long as they don't go much over +P limits, that covers their backside.
In .45 Auto, QuickLOAD says my cases (26 grains water overflow capacity) have about 15 grains water capacity under the bullet when a Hornady 230 grain FMJ ENC bullet is seated to 1.270" (the number I use for it). Each 0.100" deeper takes about 4 grains of capacity away, or 40 grains per inch. If I seat the bullet to 1.190", or 0.08" deeper, I then have 0.08 × 40 gr/in or, 3.2 grains less water capacity or 11.8 grains water capacity. If I used a 5.0 grain charge of Bullseye originally, but want to keep pressure constant, I need to reduce that charge. A rough approximation for the reduction it is to multiply the original charge by the ratio of the new water capacity to the old water capacity raised to the power of 5/8 or the power of 0.625. So, I have:
5.0 gr. Bullseye × ((11.8 gr. H
2O / 15 gr. H
2O)^0.625) =
5.0 gr. Bullseye × (0.79^0.625) =
5.0 gr. Bullseye × 0.86 = 4.3 gr. Bullseye
In reality the difference may by smaller because of the tendency of primers to unseat bullets before the powder gets burning in small volume cases. The thing is not to count on it and err on the low side if you can by assuming the formula is roughly correct, then working back up from there while watching for
pressure signs.
