The load that works for me is 43.3gr but it may vary based on your rifle. As always start low in .5gr increments with 3 rounds each and test it. I had pressure signs at 44.5 in mine, so pulled the rest as I was achieving the accuracy I want. All the best.
The same load seems to work for FMJ for me for range use with 150gr Game King.
As everyone else says, mid-40s seems to be the recommendation. I never really 'developed' a load for the 150gr Game King, just loaded 5 at 44gr and Sierra book length to check pressure and see what it would do - ended up leaving it at that:
I shot the same load at 300 yards at Bisley the other day and it produced reliable v bulls (unless I made a hash of things). That'll do for me!
I load 42 grn of N140 under a 150 grn Sierra Game King. COL is 2.800 inches. This was in a 20 inch barrel with a 1:10 twist. The GK is a boat tail whereas the Prohunter is a flat base, but otherwise the same bullet. This difference may or may not be significant. You could start with this load and vary above it and below it by a couple of grains in each step and see what works for your rifle.
Alternatively get hold of the Vit load tables (they have an App you can download for free) and start at the minimum load and work up a load ladder increasing by .2 of a grain until you find an accurate node.