Tin snips for the round hole, and an angle grinder with zip cut for the Rectangular hole only because of the overlap of the adjustable plenum meant cutting through twice the thicknesses.
A = Pie x r x r
A= 3.1415 x 4 x 4
A=50.2 sq" for 8' round duct.
Keep in mind, if you are feeding with an 8" duct, you need enough registers to to vent "almost" the same area. (Manual says slightly less, but that can be dealt with register flaps or dampers to balance flows.)
You will need Two 6" round registers per 8" supply duct.
As far as the 0.2 duct back-pressure, well see after I get the fire going. I have a big system with long runs and lots of outlets so I expect it to be lower. I'll have to play with the dampers on each run to get my flows and static pressure dialed in.
The reason the manual wants your register areas to be "slightly less" than your feeding area is so the entire system pressurizes, and flows evenly out of each register. Too many registers just let the air escape out the closest registers to the furnace, and nothing gets blown out of the end of the runs. Lots of people have cold rooms on the far side of their house for just this reason. I restrict registers close to the furnace because that air is the hottest and lower flows are needed to heat the room.That builds the required back pressure of 0.2"WC to force air (now slightly cooler...) in higher volumes out of the furthest registers.
If I go too far and over restrict the registers, the static pressure will rise above 0.2' WC, and that will reduce the overall air flow through the furnace and overheat and damage the firebox. THAT's why the 0.2" spec. is in the manual.
one 12 inch should be two 6 inch. Been working good getting nice secondary burn .
Made this stand , No one ask why I painted it green .
