I recommend break-in on a bench so you have full control at all times. Do a series of short runs of about two minutes which you end by pinching or disconnecting the fuel line and let the engine cool down naturally. Each run of heating up and cooling down is called a heat cycle. The series of heat cycles provides a smooth fit between the piston and cylinder without excessive heat.
First run, run it slobbering rich for a minute to flush out any junk that may be in the engine, then run it up to rich side of peak for one minute. If it starts to slow down at any time (sag), pinch the fuel tubing (or pull it off) to shut it down. Richen the needle a couple of clicks and try again.
After it cools down, start it and run for two minutes. After half a dozen heat cycles, try to run it at peak. If it slows down, stop it and do a few more heat cycles. If it runs a full tank at peak it is broken-in and ready to fly. The number of heat cycles needed for break-in depends on the fits of the individual engine.
There is seldom only one way to do things and break-in is no exception. Some run it in a little, then finish break-in in the air. Some do not break-in an engine.
I believe my way lets the engine be all it can be.
George