siteunseen's approach is a good way to more precisely check float levels. I manufacture and sell Float-Sync on eBay (pictured below) and have received excellent feedback, if you are interested.
Float-Sync is the best way to calibrate float levels between two Datsun 240Z SU carburetors because you'll actually be able to see the fuel level in each bowl, even with the car running. The level of fuel in the float bowls is the first step you should take before making other fine adjustments to optimize “runability,” In my experience Float-Sync gets you to the baseline as quickly and accurately as possible.
I learned, like most of us, to set float levels by removing the float bowl cover and measure the distance between the edge of the lid and the top of the float while the cover is inverted. What I've since learned is just how imprecise this approach is because many variables. Those variables include whether the carbs are being run with or without float bowl gaskets, how many turns down the jet nozzles are set, if the float valve is fully opening and closing, if the float bowl lid has short or long ears and if floats are intact and their buoyancy is the same in each float bowl. I'm sure there are a number of others.