Of course 30% from the leading edge is the correct amount but that is 100% right IF the wing is a straight one. If you have a tapered wing (leading or trailing edge or both) then the 30% is not at the same point as the straight wing. For a tapered leading edge then the CG is farther back than a straight wing and for the tapered trailing edge the CG is farther forwards. I hope I studied my physics right!

Always maiden your plane slightly nose heavy. If the recommended CG is 100-120 I will usually set it at 105 level or 110 about 5-10 degrees nose down. When you do this the plane will always not surprise you. Then as you get comfortable with the plane just shift the CG farther back.