You are correct, it has to do with static pressure drop in the duct which is a function of (among other things) velocity. You are dealing with airflow volume (cubic feet per minute, CFM) and duct area (sq ft) and this gives you velocity (CFM/SF= velocity in feet per minute, FPM). So it's not just a matter of number of take-offs, but how much air (CFM) goes into each, or, more specifically, how much is left in your main.
I'm sure someone here will give you some contractor "rules of thumb", but to do it properly you need to know how many CFM you have in your main at (or immediately after) each take-off so you know how/when to reduce the size of the main.
I'm sure someone here will give you some contractor "rules of thumb", but to do it properly you need to know how many CFM you have in your main at (or immediately after) each take-off so you know how/when to reduce the size of the main.
