loaddata.com has some loads that it says are from the Lyman #3 Cast manual.
None with a 100gr bullet though, 92gr and 122gr.
Their loads for the lighter bullet are: 2.3gr of W231 to 3.5gr. Velocities are from 730ish to 1000fps. The OAL was .980". If you are going to use that OAL, no doubt that the base of the bullet is going to be deeper in the case than theirs. If it was me, I would start at 2.8gr and go from there.
Because they are being shot in an auto, I never start at minimum. I want that slide to cycle and a case to eject every single time. Once I turned a Sig P226 into a single shot by starting out too low but still in the data, never again!