I have to agree with MGlavin that 6" will work just fine. I have a 1/4 scale cap with the throtle servo only 6" away and it works flawless. In fact it is less than 6" from the switch for the ignition. I also agree with everyone that has ever said that you want to mount it as far away as possible, just because.... You can also use a fiber optic to connect the servo and mount the servo right next to the ignition source if so desired. I will also suggest that you range check EVERYTIME you get ready to fly. Good Luck
Jeff