I'm not going to be such a nay-sayer not having any actual experience with RL15 in your intended application.
I will say that a friend has for many years used the very similar IMR 4320 in his .25/06 and would rather fight than switch!
He uses the long published max load of 43.0gr with a Sierra 120gr. BTHP Gameking. He gets a chrono'd (my chrono) 2,900 fps and ONE-HOLE groups. I mean 5-shot one hole groups (~0.4-0.5") for 5 shots from a box-stock Interarms MkX (circa early '80's) on demand, not just once when the moon and planets aligned....
A previous poster listed a start load of ~36.0gr and 43.0 as max. This sounds reasonable to me. Use normal caution in working up a new load.
Let us know how it works out. Also, there are loads listed with IMR4064, Compare these to what you're considering. (38.8 start-43.5max-2,885fps. Also Varget @ 38.0 start-41.0max-2,755fps -LEE #2nd edition). These are 120gr loads. In my experience, substituting a different bullet of equal wt. will make more difference than the 5gr difference wt. due to differences in bullet bearing surface.
To be truthful though, the slower powders will give better velocities, but, 100fps dosen't trump accuracy.