you just have to tried the ratio out ..
start at 11 and watch the temeratur of esc and motor to adjust the ratio..
it depent alot of the track your running (big / small) but with a 2,5T and 6 cell you need a big track .
how long you can turn with a 2,5T ? I drive on a big track here in luxembourg and last year somethimes I do not finish with a 3,5T and 6 cells the 5minutes runtime
these year I run on 5 cells so i need to run a 4,5T or maybe a 4T to finish the 5minutes ...