What you're saying is true, but there's no fixed rule... for several reasons...
? Most 9mm brass won't allow you to seat deeper than ~0.30". The wall thickness increases internally. Set your caliper to 0.353" and slide the tines of the caliper into the mouth of a cartridge case. You'll soon find a decreasing internal taper that limits
Min OAL.
? If the ogive shape is the same, a 147gr is a longer bullet than a 115gr bullet. Therefore the 115gr can run a shorter OAL and not "bottom out", whereas one has to be very careful with the 147gr or it will. So there's not one, single, magic OAL either.
? Also, powder burns progressively better as pressure is increased. So a plot of powder performance is NOT a straight line,
it's a curve. What you're suggesting is making a mathematical proportion, which only works on straight line graphs. Still sometimes we use this technique when we have no other clues as to where to start. It's better than nothing, but it only "works" becasue we then back off and begin at a "
Starting Load".
If you'll look at my work on the 135gr MBC coated lead RN. There was no data available for a 135gr lead bullet, so I did a proportion between 124gr and 147gr. Turns out 135gr is exactly halfway between those 2 weights. So I proportionalized the loads. The calculation was way off, but I erred to the low side and only my journalistic pride was injured.

Generally, 9mm loads seem to perform their best...
? With 0.20 to 0.25" seating depth
? With a powder loaded to about 93-95% of
Max Load for the velocity you desire
Hope this helps.