You'll need a ruler that's cut off so zero is on the edge of the stick. What I mean by that is you set the ruler on it's end, so you want to start right at zero there.

Holding the carb on it's side, place the ruler where the bowl would normally attach and keep on eye on the needle. Rotate the float so the needle closes off the fuel, and take note of the "highest" part of the float compared to where you're measuring from. You do not want the spring loaded plunger to move into the needle, so recheck this measurement a couple times. Use a small screwdriver to adjust the measurement and don't go nuts - it's easy to check, adjust, check, adjust, etc so you don't have to justify bending the heck out of the tab and getting it way out of whack.

Be sure to check both sides of the float as that can get you messed up if they are wildly different.

Craig