Howdy
I used to use FFFg in 45 Colt and 44-40 and FFg in 12 gauge shotgun.
I got tired of keeping two different granulations of powder around so I changed over to FFg for everything. 45 Colt, 45 Schofield, 44-40, 44 Russian, 38-40, 45-70 and 12 gauge.
Generally speaking, you will see an increase of 60 100 fps going from FFg to FFFg, all other things remaining equal.
Strictly speaking, the actual grain weight of powder that you fit under the bullet will depend in a large part on what brand it is. They do not weigh all the same. It also depends on how much of your bullet extends into the case.
The easiest thing to do is take a measurement of how far your bullet sits in the case when it is seated. Then add enough powder to a case so the powder will be compressed between 1/16" and 1/8" when the bullet is seated. Pour out that amount of powder and weight it. That will be your charge with that bullet and that brand of powder. If you want you can make up a custom dipper from a cut down shell casing.
The Lee dipper set is great. I use a 2.2CC dipper full of powder for the 250 grain Big Lube bullet in 45 Colt. Scrape the powder off level with the top of the dipper, do not use a 'heaping' dipper.