Cat Eye cyclocomputer--tire sizes

Hey ladies, I need some conversion assistance. My road bike tires are 650x23C (I know, I know-some day I'll get a big girl bike). The information for my new bike computer (Cat Eye Mity 8) shows only the following sizes:
650x35a
650x38a
650x38b

I programmed in the lowest (35A) but my speed and mileage is way off --too fast and too far. Any thoughts as to which one might best match my tires? I'm almost ready to live with it 'cause it took forever to push 3 buttons at the same time, jump up and down while scratching your head, count backwards from 1 Billion AND remember which button makes what work -- can't they ever make the programming instructions easy???

hugs!
~AG~

AG,
Read this thread, we were just talking about this:
http://forums.teamestrogen.com/showthread.php?t=15395