have chatted to 2 instructors about it but got different answers
That's because they haven't got a clue. Most PPL training in the UK is done with the mysterious red lever all the way forward (wasting about 30% of the avgas) and the flights are kept short.
That's why I suggested doing those two flights, etc.
One should never push fuel planning to the limits, but accurate data enables you to plan the reserves with confidence. Let's say you have established that your range to dry tanks is 630nm, then (at say 100kt) you can do a 400nm flight, 100nm to alternate, and still have 1hr in the tanks (the FAA IFR reserve rule, FWIW in this case). My own planning is destination, alternate, then 2hrs in cruise, but then I fly a TB20 with a dry-tanks range of 1300nm.
The stuff I was taught in the PPL, like "3 hrs in an Archer" is just crap, and dangerous once some pilot discovers that he can stretch it quite a bit.