Howdy
All other things being equal, you will see an increase in velocity of between 60 fps - 100 fps using FFFg instead of FFg. I used to use FFFg in all my 45 Colt and 44-40 loads, FFg in my shotgun loads. I got tired of stocking two different granulations, so now I use FFg in everything, 45 Colt, 45 Schofield, 44-40, 44 Russian, 38-40 and 45-70. It really does not matter very much.
Don's method is pretty similar to mine for determining the correct powder charge for any pistol cartridge. Put in enough powder so that when the bullet is seated the powder will be compressed between 1/16" - 1/8". That's all there is to it. If the firearm is in good mechanical condition, you will not damage it with such a load.
You have not mentioned what the firearm is that you want to fire 45 Schofields in. For the record, modern reproductions of the S&W Top Breaks such as the Schofield model and the Russian model do not do well with Black Powder. Not that it will hurt the gun, but Uberti has made design changes that cause them to bind up more quickly with BP fouling than the originals did. Colt style revolvers are much more resistant to binding because they still retain a good bushing on the front of the cylinder.