Well for mine its 7 3/4 " But it is different for every plane! I suggest that you start at the CG as per the manual(7 7/16"), then slightly move the battery pack or lead back a bit every flight.
Pateints is the answer!
Also to make it easeir i built up a whole floor with some reinforced lite ply! This made it easier to move any item anywhere i wanted in the fuse!
If you are curious i have hitec 5625 all around and a Saito 150 up front
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