Cross-chain combos cause excessive lateral strain and wear on the chain. You dont need to use these combos, similar gear ratios are replicated with a straighter chainline.
You measure gears in the traditional manner using "gear inches" not car-style gear numbers:
http://www.sheldonbrown.com/gloss_g.html#gearinch
see also
http://www.sheldonbrown.com/gloss_ba-n.html#btension
and
http://www.parktool.com/repair_help/clg2.shtml