I generally apply the first 'notch' of flaps right after pulling power back to about 1700 RPM at midfield downwind to start slowing the C150 down to landing speed (in the 65-70 MPH range). A quick look at altitude when turning base might call for another notch if I'm too high. After established on final, going to 30 degrees is an option, again depending on altitude. Never get to 40 degrees, based on student experience, when both my CFI and I weighted in excess of 200 pounds each. The C150 just wouldn't do a go around with that weight and 40 degrees. The only "hard and fast" is the first notch of flap when reducing power at mid filed. All the rest is "if I need them" based on if I am too high for that segment of the approach.....
***Reality - The refuge of those who can't handle simulation***
Cheers, Jerry (N94)
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