I’d suggest a groove diameter of at least .379. Al’s moulds never seem to cast under-sized for me, although I rarely use straight WW. It is common for my results to have as-cast diameters of .002+ with alloys that are somewhat similar to Lyman #2. I have this mould but have not yet had the chance to shoot any of the casts, which drop at .382 consistently. My 375 Win has a throat that is slightly less than .379 and groove diameter of .3775. To be honest, I don’t like sizing .004 down, so my next move is to try different alloys and vary the melt/mould temps. My plan is to use straight WW next chance to see how they measure.
I’m hoping Al starts offering optional diameters on all the group buys, if it isn’t too much of an issue. I’d like to see this mould offered at .376 as an option. For my other 375s, a .374-5 version would make a fine PB plinker.