Too much timing?

As far as initial timing goes.......
The factories had to be conservative...& probably erred on the side of caution....when specifying the initial timing.

As for the OPs situation.

It doesn't matter where the cam is installed or what the cranking pressure is. He has found the 'sweet spot' of initial timing for THIS engine, 24* BTDC. His test method was quite legitimate. If the engine & starter tolerate it under all conditions, all well & good. Some engines wouldn't, might buck the starter or ping. With this, the fix is manifold connected vac adv with an adjustable VA so that it can be properly calibrated.

OP, keep going you are on the right track.

For those who think 24* initial is a lot, Pontiacs with 10.75:1 CR, iron heads, & mild cams were driven off showroom floors with 26* idle timing [ 6* init + 20* added with MVA ]. I believe Chevs [ some at least ] had 24*.