AlbanyCO: Others may disagree, but using data developed for another 150-grain boat-tail bullet should be safe with your Sierra Gameking 150-grain BTSPs. I have never had any issues with using a proven charge weight listed for one bullet, in a load for a similar bullet of the same weight in a given rifle. I do stay with relatively moderate loads. A shooter experimenting with full-throttle loads might do well to back off a bit to start.
For example: I had a supply of Nosler 150-grain Solid Base boat tail .308" bullets a while back, and developed loads for both ought-six and .308 Win for them. This was the first time I used Hodgdon Varget, and it gave excellent results. When the last of the Noslers went downrange, I switched back to my old favorite Hornady 150-grain .308" boat tail, using the same charge weights of Varget. No changes in pressure signs could be detected. The Hornady was slightly more accurate overall than the Nosler, as has been my experience for years.
If you were going to go from a boat tail to a flat-base, it would be a good idea to decrease your proven charge by a small fraction, just to start.
To all: I agree with the general opinion that Varget is a superior medium-speed powder. I used 4064 for years with good results, but Varget meters so much better, with considerably less bridging in the measure, that I use it in .223 with 55 and 69-grain bullets, and .308 Win, .30-'06 and 8x57 with 150-grain bullets.
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