With the Hornady Reloading Manual 8th Edition, for every cartridge, it shows a starting load with a starting velocity and they increase in 50 fps intervals until the maximum load and velocity is reached.
No extrapolating required .
The 38 Special with 140 grain lead FP cowboy bullet, using Titegroup powder has loads starting at :
3.0 grs. = 700 fps , then moves up
3.3 grs. = 750fps
3.7 grs. = 800 fps
4.0 grs. = 850 fps
4.3 grs. = 900 fps
to a maximum load of 4.7 grs. = 950 fps

I find with the Hornady manuals it helps me verify those middle of the road loads I usually find best. If you don't have one of their manuals , it's a good one to get.
Gary