A comment regarding flap use. I recommend you read the aircraft's pilots operating handbook or flight manual in the section labelled Normal Procedures and see what it says with respect to flap settings for a "normal" approach. Based on my rather sketchy memory of the 172M manual I think it will say something like: "flaps 0 degrees to 40 degrees as required". That means you may use any flap setting you want and not put the aircraft out of limits assuming you have the airspeed under control.
Of course, your instructors word is LAW when it comes to operating your aircraft. Does your aircraft have the 3 position flap toggle switch (up travel, neutral and down travel) or the notched gate (a jagged slot with kinks for each flap setting)? If it is the former I fully understand your instructors motive for getting you to use 20 degrees. I am blessed with big hands so I use my right thumb to push and hold the throttle fully open while using the fingers to toggle the flaps up when doing touch and goes in the clubs 172M. A third hand would be nice to have in those situations but we make do.
