LRP X12 10.5 gearing in an SC10

[Team No Idea]

Hello chaps......right im a tad lost in the land of gear ratios. Bit too complicated for me!.

Ive fitted an LRP X12 10.5 to my SC10. I have ran the truck in the past using a Novak 6.5 motor and had a few overheating issues. I ended up dropping the the spur gear to a 78 tooth.

Now here comes the complicated bit.... in the motor instuctions it says to run the 10.5 at a ratio of 9.0:1 for "truck off road". erm so any ideas what pinion i should be using? . I did have Gear ratios explained to me a while ago but for the life of me i cant remember a damn thing about it!
any help would be fabbo!

Thanks!