you really should do ratios based on teeth.
Each gear has a tooth count stamped on it. I think it is 24 and 48, the gear ratio is still 1:2 or 2:1 depend on which gear is the drive and which gear is driven.
Here is an excellent Gear ratio spread sheet that I got from another team on Long Island in New York.
It was created by Frank Saladino - Team 353 -
www.pobots.com
It has been a very valuable tool for our team. (yes I know this is a bit off topic for helical gears, but you'll love it when you see it) :-)