You need to be a little more specific on what is an "Advancing" pose? Is it a man with a rife on his shoulder or is it a man with rifle at the ready? Anyway, screw the percentages, for your usual 10 figure sprue, her is the breakdown as I see it;
Advancing(rifle at the ready): 2
Misc pose(reaching into cartridge box, casualty, kneeling firing, bayonetting, etc): 1
This is of course considering you will offer a command set later.