This sort of "SOP's" sounds a bit funny to me...
You have (hopefully) a POH and there the speeds are given by the manufacturer and thats the way the aircraft should be operated. The first approach to a very short runway but in the limits of the C172 will end with an overrun. Your described aiming point is NOT in the same spot at the same speed with flaps up or full down, as the attitude of the aircraft will be different.
Sounds maybe harsh, but your INSTRUCTORS will need training how to fly a C172 and then should train you properly. Instructing everbody to take 70kts as reference speed for all circumstances, they assume all the students/members of the club are to stupid to remember a few more digits and this is not a trustful manner towards the clientel.
Finally, this may end after an accident (which I hope will never happen) with a refusal to pay , when the insurance finds out, that the plane was not operated according POH.