I too have just built a 338 lapua mag
I have had good luck with the following:
250 gr match king, 96.5 gr of Rutumbo, cci lg rifle primer
300 gr match king, 101 gr of US869, cci lg rifle primer
both loads key holed the target at 200 yrds
1 shot at 7 o'clock in the 8 ring
the other shot at 4...