You can measure the height quite safety with the engine running. All you are doing is removing the jet cover and extracting one of the main jets. As the main jet is not being used at idle this has no effect on the engine idling. You can then measure the distance from the top of the hole for the main jet to the petrol surface with a micrometer and a torch. Should be 25mm +/- 1mm, however mine run best around 28mm.
If your running an electric fuel pump you can just remove a main jet from the three carbs, turn on the ignition so the pump tops up the fuel, then switch of the ignition and measur