Ok ive worked out the internal ratio and have an exact ratio chart to the Kyosho manual and is 2.214:1 and used a 100t spur and 17t pinion so gives you a range from pinion size 10 to 25 so should be enough for all.
So if you use a gear ratio chart like the one rondoolaa supplied
http://www.gearchart.com/index.cfm?f...n=chart.create
then game on. One less thing to worry about