The 165 gr GMX Superformance factory load that i pulled bullets from , had 44.4 gr of powder +/- a tenth or two. That was from the first lot , that grouped excellent, that i pulled down. The second lot,that i pulled bullets, had 45 gr of power +/- a tenth. I copied the 44.4 gr load using 168 gr vld and 168 gr Horn. HPBT.......both grouped around .75 @ 100 yards without trying very hard. (Factory load, grouped .50 to below.25 @100 yards) .......I' m aware that what we can buy and what hornaday loads factory with is/may be a tad different, but ive not ran into trouble copying their factory loads.
My advice is to pull a bullet and weigh the powder and work up to that weight. Make sure all powder is out of the factory load. Those 165 GMX loads are heavily compressed and all the powder hasnt just dumped out.
Good luck......im a big fan of superformance powder. It has worked real well for me.
Edit: Im interested in what you come up with. Please post your results.