For toughest don't read ogre, it's more to with the standards I expect. I always expect them of myself so I would like a similar attitude in my students.
There are times and places for 'fudge' factors, approach speeds are not one of them. Too much speed is a bad thing, a bit too much fuel is a good thing.
Having a clear and full understanding of any subject is essential before you can start to take a few liberties. I'm all for having SOP's, but blind adherence to the rules with no thought as to why we do what we do is in my opinion lethal. SOP's cannot cover every eventuality.
For instance if you have worked out a wonderfully accurate fuel plan, but fail to check how it's going en-route, then the plan is useless if you don't notice an abnormal fuel flow and end up in a PFL.
There is NO excuse for ever running out of fuel.
Trying to plan fuel usage for a lesson is a total waste of time since fuel flows and power settings will often be changing constantly, will a navex use the same amount of fuel as a session of ccts or stalling? No way, this is why we'll usually use 1/4 a tank per lesson usage, filling up whenever it goes below 1/2. Leaving the final 1/4 as a reserve. Is this accurate? No, is it safe, Yes and there is no room for confusion. That to me is the most important thing.