FWIW.
I made a wooden dipstick long enough not to tilt/fall into my Rans S6 wing tanks.
I paint mine with gloss black paint. Then the fuel wetting shows up clearly, but quickly dries off again as it doesn't soak into the wood, so repeat depth checks are easy.
The S6 tanks when filling have different cross sectional areas as they approximate to the wing profile, so for me the best setting up method has to be fill and mark in 5 litre steps. It's now served me safely for 1/2 dozen years
mike hallam