Understand I'm NOT any kind of expert and make no claims to being knowledgeable on this...just report what I've done & u can either gain something from my BS or not.
I've had good luck with 3/4 inch on two medium sized saws..a 365/372 and 820 Jred. But I added x-sectional area to a 2094 and it pulled harder..my bet is the 85 percent is a good safe place to start but depending on a lot of other factors ranging from displacement to port timing and size other sizes might work better. Here are two examples of ones that did work better after the muffler mods:
This a 94cc GTG Play saw...so I wasn't as concerned about the exit location and direction ...just noise and power...this saw howls! The exit is high enough it takes some real big wood to block it when rocking into a log...but it still will.
Runs way better than it looks. Has widened exhaust and widened & slightly lowered intake (.100in) along with some other stuff to the transfers and intake horn & carburetor mount..and over 190psi compression. (I know! I know...but it made a difference on this one!) Was looking for x-sectional area...an experiment that made a positive difference in this case...but probably NOT in others. Have to be careful when cutting to not run the exits right into the wood! NOT a practical mod for a work saw.
This next one is for an old J-red 820..80cc or there about..absolutely about a work saw...had to extend past chain brake and also directed off to the side where I wouldn't block the exhaust when in real big wood. This saw runs a 28 inch bar often and is a torque oriented design so I saw no reason to go nuts with X-sectional area. The mod did product more power...not my best one but I was (and still am) learning was was best for work saws.
This IS a 71cc work saw (despite what it looks like)...so I was concerned about real world use and directed the flow off to the right and down a bit. Can roll that right to the wood and not lose exhaust flow. Also only lightly modded so 3/4inch seems to be fine...this is along the lines of a configuration all my work saws will have from this point on.