Hi Paul.
1. It should be 13.2 mm from the sealing ring on the carburetor body (where the float bowl seats) to the top of the float, with the float slightly lifted so the needle is against the seat. Whether it is legitimate to turn the carburetor upside down depends on whether it is a light float or a heavy one - you judge that in each case. Usually it is OK to turn it upside down.
2. #92 means 0.92 mm jet. That is 0.036" For comparison, a GXV140 has a #45 main jet: it is a much smaller engine, compared with the GXV390. There are two reasons to check it. First, it might be clagged up with gum. Second, some previous tenant might have changed it or soldered it up. Find a drill bit that just goes through, and measure the drill diameter.
3. The idle mixture screw (or pilot screw in Honda-speak) is the fine adjustment of the idle mixture. The reason to check how many turns it requires to get the best and fastest idle, is because it is one of the ways to tell whether something is wrong up-stream from there. If you get the best idle somewhere near the specified setting, all is well with the entire idle system. If it just keeps getting better as you turn the screw anti-clockwise, something is badly wrong in the carburetor.
