fyi, the csa consideration at the top of the riser
should be conceptualized as a sort of chamber around the riser, between the top of the riser and the barrel...the first point at which air leaves the riser and enters the barrel. so to be consistent with system size, csa for this area is calculated using the length of the top of the riser (inner circle rim).
if you do feel the need to calculate the gap based on csa, the proper way to do it is to find the length of the top of the riser...so in an 8'' system with a circular riser, the length is
pie x diameter...3.14 x 8 = 25.12.
the csa in a circular 8'' riser is pie x radius squared...3.14 x 4 x 4 = 50.24.
then the gap is found by dividing csa over length, so 50.24 / 25.12 = 2.
in a square riser, the length is simply the sum of all four sides...but remember that it's best to use calculations for a circle that fits inside the square...and also, as Glenn pointed out, that more space is better.