Floor channel and stub sizing

I can't find a chart or formula for sizing the floor channel and stub, only for the P-channel.
Can anyone point me in the right direction?

Hi William;  
Peter has it on his site.
https://batchrocket.eu/en/designs#floorchanel

The only thing I can find there is a chart that lists P-channel size for different riser sizes.

I've read on Donkeys that P channel csa an Stub csa are basically the same, and the horizontal floor channel is twice that.
Extrapolation from P-channel height and width gives a figure of 4.7% of csa, but I'm bad at math,so that might be totally wrong.

Based on the numbers on the chart the ID of the vertical stub would be 1 5/16" , so between 1.25" and 1.5" nominal pipe size.
Am I getting this right?

Hey William;
Math is not my thing.
2.5 x 1.5 for 6"
3.5 x 1. 75 for 7"
2 x4 for 8"