I made a gear chart for the DB01 over a year ago here:
http://www.oople.com/rc/photos/cars/...-gearchart.pdf
I just go by reccomended ratios for whatever type of motor it is then fine tune it from there depending on speed / how hot things are getting etc. With my 5.5 bonded brushless I was running ~13.00 overall ratio and ~10.50 with a 6.5 sintered.