DIY "recumbent" trainer I have some back issues and my doctor recommended to use a bike with a chair  instead of a seat post. I have a old bike that I used for a trainer. I plan to cut the front part of it, turn the remaining part around (so the pedals turn in the right direction) and build a seat in front of it.
I'm trying to build the seat in front of it, to use as an indoor training rig.
Here is a simple schematic:

However, I'm having trouble figuring out the exact geometry of the seat. Can anyone recommend some resources/plans ?
The D size is fixed, I cant cut more from the bike seat post to lower it. I'm interested in determining the A angle, and the B/C sizes.
Edit:
Just went ahead and cut everything from the bike except the rear wheel support and the pedals. First mistake: cut the D part too low, the pedals hit the ground.
However, after cutting, and sticking a 20mm board under D, it's almost usable with a kitchen chair.  Only problem is that the trainer gets away from you when you push on the pedals, and lack of back support.
HTH
Atomic Zombie has a bunch of DIY recumbent bike plans, many of which are based on cutting up existing bikes. This one seems like a reasonable starting point. I think that sitting in a regular chair might not give you full leg travel--recumbent seat bottoms are pretty short by comparison.