What you seem to be looking for is an absolute, hard, empirical, perfect, minimum tolerance type of number to go by. Sorry to tell ya, but there just ain't none like that to be had. Hobby, not rocket science.

In this hobby, quite a bit of it is trial and error, done from prior experience with a similar product/installation, or just "looks right." Basically you either change mufflers and use something that lets you feel comfortable, move the current engine/muffler combination, or become one of us ol' time pioneer hobbyists and give it all a shot to see how it works out. You never know what you can do until you try.