Help with backspacing on 13x7 for 88 CRX

[Buxton <buxtons@yahoo.com>]

I am looking into running some 20x8x13 cantilever
tires for track events and autocross on my 88 CRX. I
want to have Diamond racing wheels made for this
purpose. All I have at the moment is a 5.5in rim to
put a tire on and check my clearance but with that
small of a rim I can not be sure how far out/in the
tire really will be. So I call out for info from of
the experts rinning this kind of set up.

Thanks for any help, big or small 

Bob