I dont do any equations i just "fly the plane" you can feel it...... Never had a problem yet!
However, I was taught not below 65kts in a Cessna 152 but my examiner told me to do a shortfield at 50kts and that worked fine too
Its been many, many years since I read a POH for 152 but 65kts seems high to me, granted that is based purely on my calculation method that may be wrong but allow me to explain my logic and I would be interested to see if someone could pick it apart, and if so it would change the way i figure my landing speeds on my plane, so here goes.
If your short field speed is 50KTS which which would represent a multiple of 1.25 of stall ( more experienced pilots use 1.2 but i am not there yet) would mean a stall of 40KTS, so a normal landing should be flown at no more than 1.3 of stall which is 52k. I would assume that anything north of that speed and especially 65kts which is a significant addition would be unnecessary and may in fact provide a greater chance of a long a float or ballooning in less experienced flyers.
Edit: I just thought i would add the reason i do those calcs is not because i am a mathematics genius ( far from it) it is because if and when i fly planes where the POH does not list approach speeds (like my POH does not), but always lists a stall and gross weight I can figure out a guide for my speeds prior to take off and without feeling its a trial and error thing especially if its the first time i have flown the plane