I have used several different sizes but this one I believe is .150" or just slightly bigger then an 1/8" Some guys think it's too small but with the proper stock fit the hole lines up just perfect for me. It's also deadly precise for longer shots. It's easy to hit pop can or clay pigeon sized targets at 100 yards.
I have spent considerable energy debating in my mind what size to make this. It's staying as it is for now, it's easy to make it bigger but once cut I cannot make it smaller! I think when the stock fit is perfect your speed to sight picture is very fast regardless of a small amount of change in appeture size. If the fit is awkward then your going to have to fidget to line it up just right.
If there is one thing I leaned the long hard way over my career it's that Proper stock fit is just as important as cartridge, scope, rings, etc etc. the stock must fit to have instant sight aquisition when you through the gun up to your shoulder. I think it's why the British double rifles were built exclusive for the person buying it.