Hi,
I instruct on the C150
On base leg, reduce power to 1700 RPM or less, hold the aircraft level (ie do not let the nose drop or lose height) and once within VFE (white arc), take Flaps 20 and allow the speed to come back to 75mph (approx 65kts). What you should find with that power setting is that by the time you have the flaps to 20.. the airspeed will be about right.
Once you have the speed stabilised and the aircraft trimmed you can concentrate on setting up a stabilised approach at that speed(75mph)
I emphasise not letting the nose drop when you reduce power.. it causes no end of problems in getting the approach stabilised.
The aircraft will land quite happily at FLAP20, no real need for FLAP30 or more unless you have a very short runway indeed.
Hope this is of some use