About 1/4 to 1/2”, but with a side by side it very much depends on the original purchaser. Most English boxlocks were made in Birmingham and then sold to the trade, partly or fully finished.
You would go along to your local gunshop “Bodget and Bang”. They would have guns in various states of finish and they would then finish up a gun for you - ie cut stock to length and cast to your dimensions. And deliver to you in a couple of weeks.
With most side by sides it is or was quite easy to adjust cast and drop by heating the hand and then bending it across and letting it cool. I say “was” as new wood is much easier to bend, than old wood. And with old stocks you do run the risk of hairline cracks etc - which whilst wouldn’t let go in normal use - could cause issues when trying to bend.