Short answer to calculating stem length with different rise is here as long as you know the angles. http://www.brightspoke.com/t/bike-stem-calculator.html
I have found that manufacturers can vary slightly as to how they measure stem length but usually it is length from the center of stem to a parallel plane of the handlebar clamp center. 70mm is already fairly short for a stem and you could probably go to a 50mm without much difference in handling but really hard to know without actually trying it out. Even shorter may be a possibility. As a very general rule the shorter the stem the more twitchy steering becomes for most bikes but not always the case. Most bike shops have boxes of cheap stems that were traded from new bike purchases and it might be worth asking if you could try some out before buying since they usually can't get rid of them fast enough.

Ooops. Just noticed two others beat me out with good answers while I was writing this but hopefully this will help as well.