Today I worked on the base mount for that bench. I chose a 20 degree slope (taken from that guide) as I found it comfortably and it’s a round number too.
I fumbled a bit with some pipes but the cuts and copes to build it properly are more demanding than the time I had, so I settled on some channel iron. The math is a lot easier, I just needed to offset one cut by (channel width) * tan(20°) or 2.6 cm.
I’m very pleased with how I managed to make this weld around the corner in one pass: