Depends on the style of gun you're building.
I try for just enough down at the breech end to get a lock bolt between lock and barrel, between 3/16 and 5/32. But again,it depends on the style gun. Some use only one lock bolt.
A big chubby jaeger could have 1/4" web, and a little Lehigh or southern gun could have 1/8"
How do I get a precision web? It's real easy when you know how: Draw the plan on the side of the stock, and send it to Dave Rase.