On my Flyingthingz lawnmower, my CG is around 7" from the leading edge of the 'wing'.
It seems to fly ok.
This thing is never going to fly all that great.
It looks fantastic in the air and on the ground is a fairly convincing lawn mower.
The kids always go nuts when it is flying including the big kids.
It is always 100% of a handful to fly and requires my total focus. It's all I can do to get it to climb and I must be ginger on the controls. I fly at full throttle only and landings are about just slightly above half throttle.
If I were to do it all over again, I think I would put a much larger engine.
Good luck.