Aeronautical engineering aside, I have always balanced my planes (at least 50 planes over 30 years) 1/3rd back from the leading edge at the root with an empty fuel tank unless the plans indicated otherwise such as on a biplane where it's normally near the leading edge of the lower wing.

I always had good results without calculating 25%, 27.5%, 30% etc. Unless you are using the datum line, arms, weights, and moments to mathematically arrive at the CG, verses the "two fingers under the wing" method that modelers use, I doubt that you will come out at exactly 27.5% verses 30 or 33%. Your fingers are not that accurate but fortunately, they dont have to be. I doubt you could tell the difference between a plane balanced at 30% and 33%. Some of you are worrying too much.

The only plane I've ever had a CG related problem with was a new Tiger Moth that I knowingly flew tail heavy many years ago because I was at the field when I remembered to check the CG, did not have anything to add as nose weight, and I wanted to fly! Everything was fine until I stalled it turning final and could not recover from the ensuing spin. STUPID!!!

Yak