XRAY - Model racing cars

LRP say FDR 6.5 for mine 10.5T engine.
If I use the gear chart from Xray and pitch 48, then it will be 24t/82t = 6,49 FDR or 21t/72t = 6.51 FDR thats the closest I get 6.5 FDR.
Is it the same which of this options i choose, or is it any rules I should follow?? If I later find out I need to adjust the FDR little bit Up or Down, should I then changes the Pinion size eller Spur Gear size? I can see the pinion is cheaper than Spur gear.
In the beginning when Im new and dont know the car very well, and I wants to adjust the FDR little bit up or Down, in how big steps should I adjust? Should I go for pitch 48 or 64?

Even when the FDR is the same, the larger spur option will always give slightly better acceleration for some reason.

Skiddins