Sounds like the plans recommend 165mm, which is 6.5" I'd start out there. With your set up you may have to add some nose weight to get there. I'm putting an OS 1.20AX one mine, tank in the forward position. I will mount the throttle servo up under the cowl and even the battery if needed to get the CG right without adding lead. This plane has such light wing loading and large control surfaces that I will not be worried during the maiden flight no matter where I end up with the CG.