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.
If you are making a long flight to a short strip, you might want to calculate your approach speed according to your expected landing weight. The 30 flap looks after extra energy and 40 is much better. Extra speed in a Tiger with small flaps is much harder to get rid of as I found out
