It's minutes of a clock rather than degrees so yes 30 mins is 1/2 a turn. With Cadet engines 2-5 minutes can make a lot of differance.
One way to get your carb roughly right is to start it on the stand (at home). Set the L at say 40mins and H at 75mins. Let the engine warm up and then blip the throttle a few times. Then with no throttle, check that the engine is just ticking over. If it's racing (wheels turning) screw the L jet out 2-5 mins or until engine stops racing. Now blip the throttle again a few times to check that the engine is responsive from low revs. If not come out another 2 mins.
Next run the engine at full revs but only for a very short time (2-3 seconds). Check that the engine revs out fully. If it's too rich you will hear it not firing every other rev. This is called 4 stoking and means you need to lean the High jet to get it to run cleanly.
For a novice these setting should work ok but as he builds up speed more fine tuning will be required. Get the driver to describe how the engine is coming out of corners (ie for flat spots) and how it sounds on the straight and make small adjustments as appropriate.
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