Hi all …do I measure the 21mm  float height from the lower rim of the casing ie the underside of where the gasket sits or from the top of the gasket? 
many thanks in advance 
Tim Steve

Personally I never remove the gasket as with my luck it'd break & not have a spare. I'm not sure what they say officialy but they normally  allow a bit of leeway in there measurement plus or minus each way. Sorry, no help am I. I mostly just go by guesswork so not good.

Method is different depending on year and model.  Early models sometimes measured down from needle jet.  Plus Yamaha specs can vary, one of the Factory Service Manuals shows multiple specs for the same model in different parts of the same manual. 

I stopped measuring a long time ago and just set the float arms parallel to bowl gasket surface when float needle closes.  Never failed me.  If you go by spec, make sure when you're done the arms should be parallel or nearly parallel to gasket surface.  If not, then the spec you used and/or measurement method is incorrect for your carb.

It’s 21mm with a tolerance of 2.5mm and you can measure it from the gasket surface (so on top of the gasket). The gasket should be 1mm, (at least mine is) so what ever you measure so just subtract 1mm.