For the unknown angles I get inverse tangent (7.5/1.75)=76.9 degrees for the top one and inv tan(8.25/3)=70.0 degrees.
The other point I recall about V belts is that when they're bent around a smallish radius their sloped sides bulge out. This enables them to grip the flanges of the pulley more effectively than a simple wedging action would. Now, I don't know if wear on a V-belt will affect this extra grabbing ability, and the changing effective radius of your variable pulley will add complications as well.
Personal note: I have learned a great deal about power transmission in highly-stressed machinery. I attributed some odd behavior in mopeds and motorized bicycles to changes in technology and material. However, my short but observant spouse suggested that I factor in a significant increase in gravitational force upon my no-longer-so-youthful body. "I'm surprised it got up that hill at all," said she.
she is an amazing cook, so it's not entirely my fault.