Peter,
Just try to get the top flat surface of each float half parallel with the carb top cover face and adjusting to maintain gap between 7.5 and 8 mm before gasket. Gasket will be just over 0.5mm, say 0.5mm when top screwed down.
If the spring loaded damper ball in the needle valve is not too tired then this should support the float even if carb top cover fully inverted. Before fitting gasket and re-assembly I just concentrate on getting each float half set similarly with each float top surface parallel with carb top cover face. I do this by eye and when I have measured it comes out at between 7 and 8mm (i.e. before gasket)
Have just split my spare carb - working OK a few weeks ago and measured float height ......