Slack

How much throttle cable slack do I need at the top of the carburetor s

Just enough so that with the engine idling you can turn the handlebars from lock to lock and the revs stay the same.

I've always set all cables with just enough slack at the lever or twistgrip that you can feel it's not up against the stop. If moving the bars stop-to-stop changes cable tension, you need to re-route the cable until it doesn't.