I want to check carb float heights on my NC24 and I am trying to follow the Haynes manual for NC30 which is fairly non descriptive. I assume the 6.8 mm height is measured from gasket face of bowl to curved portion of float when carbs are inverted if that makes sense. What should I look for in reference to float needle, does this need to be fully out when floats at 6.8 mm ?

Yes, measure from the gasket surface to the highest point of the float but don't invert the carbs. Instead tilt the carb assemblt to about 45 degrees sideways so that the float's tang just touches but doesn't compress the needle spring. Best is to use a card with a portion of the desired float height cut out or a float height tool.

Also check the needle tip fo deformation and the needle spring for smooth operation.

Did this today on mine using the cardboard technique. All were near as dammit to 6.8 mm. Peace of mind whilst the carbs were on the bench.