Here we go... first the bad news, the rest of the world pretty much doesn't load 3/4-oz in 20-ga hulls, so you're going to have to make due with the wads that are available for 7/8-oz loads. You'll need something to take up the space in the shot cup, or your crimps won't be satisfactory. I'm gonna assume you have common Remington RXP20 or Winchester WAA20 wads or their knock-offs. Get some card stock and a 1/2" wad cutter (now on sale at Harbor Freight) and start punching. You'll be adding paper to the bottom of the shot cup (up to a 1/4") to get the crimp right -- you can do a dry run to see how much it takes to get the shot to the top of the cup.
For 2-1/4 Dram Eq. (1,200 fps) 3/4-oz, 20-ga, reduced Skeet loads:
Remington Shells
Rem 209P primers, RXP20 or WAA20 wad with filler, 15.5 gr of Unique
Federal Shells
Fed 209A primers, RXP20 or WAA20 wad with filler, 16.0 gr of Unique
This is from an Alliant Reloaders' Guide and they round their loads to the nearest 1/2 gr. I suspect the 16.0 for the Federal hulls can be reduced a to 15.9 or 15.8 without any ill effects. My preference would to be to start with the Remington hulls and keep your Feds for your 7/8-oz loads -- easy to tell them apart that way.
Happy shooting.