It all depends on the engine manufacturer. Idle for a Honda is around 1,200 rpm, worst engine was the Tecumseh which idle around 2,100, Briggs was around 1,700 to 1,800. We would set the fun kart clutches to come in at 200rpm above idle but once Tecumseh started to provide engines for the industry then ALL clutches out the door were set to come in at 2,300 rpm because we had to make them for the worst case scenario.
Racing is a different animal because you want to set the clutch engagement to 200 rpm below peak torque curve of the engine. The engine builder with his dyno could tell you where that occurs. My rule of thumb when you don't know peak torque is top rpm divided it in half and set the clutch up at the half way point--so if you are turning 6,100 rpm then dial in the clutch to 2,800rpm and you will be very close to where you want to be.