In general, I would rank order from most reliable to least reliable how I regard load data this way:
- Data from the powder manufacturer (Hodgdon's online reloading data center is terrific, btw.)
- Data from the specific bullet manufacturer (if I have it, although I am never comfortable with Hornady's published loads because it varies so much from other sources).
- Data from Lyman's manual.
- Data from Lee's manual.
- Data from those caliber-specific "loadbooks" (e.g. Loadbooks USA "9mm Luger" Reloading Manual)
- The Hodgdon website lists 6.7 to 7.0 gr. at 1.125" for a Speer Gold Dot Hollow point.
- As far as a 9mm 115 gr bullet, my Hornady book only lists the HP-XTP (1.075") and the FMJ-RN. It shows a range of 5.7 to 6.4 grains
- The Lyman 49 book shows 5.7 to 7.2 gr at 1.090".
- The Lee book shows 6.7-7.0 gr at 1.125" (obviously the Hodgdon data).
So I would first seat this specific bullet in the case so that the truncated cone profile "looks right" in terms of the amount of shoulder showing above the case rim and see what length that yields. The I'd start the powder min charge to reflect the length.
I assume you are simply loading for range use here, so there is probably little point in figuring out a max load. Start at the min and if you get good pistol cycling and accuracy, you are done.
IANAL, YMMV, use at your own risk, etc.