You are correct, I must have messed up the Donek angulation guide - the 33 measurement must have been at the inserts and not the waist (I used 85*). I'll remeasure to be certain.
Your boot sole and stance width assumptions are also correct, I have a riser but not sure what thickness.
Assuming a waist of about 29cm, what would be the drawbacks of going that wide?
I have some wide surfboards and it's harder to bury the rail (something with a wide base is harder to tip over than something with a narrow base). But with snowboards you are strapped in, and being tall with enough leg strength I have never had a problem putting a board on edge (my equipment gives before I do).