I dont know the answer for the KX500, but on many carbs the float has a seam that should be parallel to the gasket surface when the carb is inverted.
On a red bike I owned the measurement was from the gasket surface to the botttom of the float with the carb inverted.
I didnt take the time to find the spec, but once you know the diminsion, and once the carb is inverted, you should be able to figure out the location for the diminsion.
The actual measurement or adjustment should not change with time, however the float can sink, or become waterlogged ( gas logged). But most likely the needle and seat will have debris or wear causing the overflow.
Post the spec when you find it please. John