by scythe » Thu Jul 06, 2017 8:27 pm
Ideally you want as much open area in the sides of the riser as you have in the ID of the riser so it poses no restriction.
IE if you have 20 square millimeters in the ID you need to have 20 square millimeters of open space in the sides.
Riser heights dont set the bath depth, correct, the downcomer height above the plate does. But the rise still should be higher than the downcomer +/-5mm
You could just mill it like you have marked out but the middle bit does not need to be as thick as you have it, just enough to get a thread in is fine.