I consider an engine to be broken in when the following occurs:

Engine can maintain a mixture setting at wide open throttle that is 400 rpm shy of max lean rpm without overheating or wavering.

and

Engine maintains a smooth reliable idle and transitions to full throttle without any problems.

If this occurs after running an engine for 1 tank or 5 tanks is dependent on many factors that I won't go into.

If you engine is oversized for your plane there is nothing wrong with a break in flight. Just be warned that you should fly at full throttle the entire time and that if you go to idle you may lose your engine. I usually richen my mixtures just past the 4-2 stroke point and go fly realizing that power is marginal and engine is not reliable if slowed down below wide open throttle. I usually deadstick her in for a landing when engine runs out of fuel.