take off

So basically,, where did we get the 50 Ft term from? Are most trees by runway's approximately 50 Ft? Or is there something deeper I'm missing?

For all my piper fellows. A year ago I bought a 1979 Archer II, did pre buy inspection and everything seemed ok. One time on take off I took a look at the RPM gauge and noticed the it was marking at around 2300RPM, I got a little worried thinking it should read at least 2500RPM full open...