Viht N160 and 6mm 70 gn
I use 48gn Vihtavuori N160 with 70gn Nosler BT in my 243win, RP or Lapua case, CCI200 primer, I get slight mark on the case head of the Lapua at 48.5gn, but RP cases are OK at 49gn. I get approx 20fps less velocity in RP vs Lapua cases (3405fps RP 3425fps Lapua both at 48gn).
The Vihtavuori manual doesn't list 6mm Rem, but for 243 gives 70gn Hornady SXSP with N160, 46.1gn 3005fps, 51.2gn MAX 3451fps, Lapua cases 24in barrel.
btw Lapua factory loads use Vihtavuori powders, and are pretty hot, I have had extractor marks on the brass of their factory 100gn 243 load, especially on a summer's day.
This doesn't help with your 6mm, but you know you're near max - with REAL data from your gun, as well as book data......
I read somewhere - can't think where - that the 6mmRem has about the same powder capacity as 243AI, I've read you can get 3500fps with the 70gn out of them, but I think it's with N165.
Best of luck, Mike