I've always been intrigued by solar heat. I found a nice site that has good info. Dunno if this has been posted here before or not, but thought I would share. Be sure to read the "rules of thumb" section.
http://www.mobilehomerepair.com/article17solar.htm
Good build.
Under the rules of thumb. 20% sounds reasonable, this is the same as most passive solar values. I don't know about the rule of thumb always being at 62 degrees. I would think it is different for different areas. It looks like it is a reprint from Rodale from 1981. I always found Rodale press to be a reliable source of information.