I use an old spoke that I measure to the desired length and bend. Bending allows the spoke to be placed against the crown and go exactly to the depth I need it along one edge of the leg. I use the threaded end and usually measure it 5-10mm longer than I need. This allows me to place the oil height accurately by saying that the top of the thread is 50mm and the bottom of the spoke is 60mm and using it like an automotive dipstick I can see exactly where the oil level is by looking at the threads and where the oil has stuck to them. Once set and exact this tool allow reproducible measurements and can be kept in our toolbox for future use greatly simplifing rebuilding.