Depth obviously depends on how thick the butt is. I would suggest the depth be at least half but not 2/3 of the buttstock width you have to work with. Somewhere in there. Be careful but not timid.
The contemporary idea today is big enough for a Snickers bar
. However just like everthing else today the last Snickers I bought was about 25% smaller than the previous one ( same price of course ) so we''ll ahve to come up with a new standard.