Take out the aircraft manual and work out your gross weight with instructor and however much fuel you have aboard.
Approach Speed = root(actual weight)/root(gross weight) x Recommended Approach Speed (which is at gross weight)
So if you are 300 lbs below a gross of 2300 (depends on your model),
we would get 60.6 kt instead of 65 kt.
As tecman says, that's a good exercise, but you've got to be careful with what kind of speeds you are using. This calculation only works with calibrated airspeed (CAS), while POH speeds are always given as indicated airspeed (IAS). The difference between the two can be substantial. A C172S has a 10 (!) knot difference between IAS and CAS at stall speed with flaps extended. (Less at approach speeds.)
So to perform this calculation correctly, use the airspeed calibration tables/charts in your POH to convert the approach speed to CAS, calculate the new approach speed using the formula above and then convert back to IAS.
Alternatively, you could use the stall speed for your actual weight (from the POH) and multiply by 1.3 to get the appropriate approach speed. This is how the POH approach speed is calculated in the first place. (Again be careful to distinguish between IAS and CAS!)