"Formula" for hand cut mortise depth?
I'm doing my first project with hand-cut mortise and tenon joinery. For the most part, it has been surprisingly easy to cut the tenons - not to say mine are super clean or great, just that I thought it would be even worse than it is. However, one thing I didn't realize was that there seems to be a point of diminishing returns in how deep I can chop the tenon in terms of being able to chop and lever out waste wood with the chisel. At a certain point, you naturally run out of room to angle the edge in or lever back without dinging up the shoulders (is that the right term? - the short ends of the rectangle that are 90-degrees to the grain) at either end. Of course, the more narrow the tenon is, the less room you have. So, you have more room to operate in a 1 1/2" mortise than a 3/4" one.
At the same time, I figure the deeper the tenon, the more gluing surface there is and the stronger the joint. But at some point, you just can't cut the mortise any deeper without a mortising drill, so the tenon then has its corresponding limit. So, is there a particular formula or rule of thumb that should guide how deep a mortise/tenon needs to be for a given width?