Traditionally, checking a float level is a measurement done with (usually) the carb off the bike and orientated so that the float sits gently on its seat. Dellortos are done this way, but there's no official float level spec given for the Mikuni in any Ducati manuals.
The way I was told to do it (not sure who by now) was to put a clear hose onto the float bowl drain outlet and hold that hose up the side of the carb to where the boss cast into the body is. On the LH side as shown by the yellow arrow in the photo below, the boss has a little nipple. On the RH side there is a dimple. You need to get all the air out of the little piece of tubing, and you'll often see a little air pocket just next to the drain outlet. The easiest way to achieve that is to remove the tube from the drain outlet with the drain screw open so that some fuel spills out. Without doing that, I have found the reading is always inaccurate.
What you are looking for is shown below. I don't think I've ever checked one that has been wrong. Certainly not one that hasn't been played with anyway. If someone has been in there trying to make it right it's usually not.
You do need to make sure the bike is vertical though. if it's tilting even a little you'll get high and low readings side to side.
The annoying part is that you need to remember to do this before you pull it apart.