"Accurate fuel management in light a/c" You're having a laugh aren't you! As accurate as a Cessna fuel guage?
Performance issues? As long as you are within limits and you have done your calculations properly (with the properly applied fudge factor of course) then where are the performance limits? How many old machines will meet the POH limits? Not a lot I reckon.
I assume that you fly the same a/c all the time. I can assure you that when you have an aging fleet of machines they are all totally different and the POH can often be totally irrelevant.
So for the 'average' PPL I advocate using some stock and sensible figures. ALWAYS take more than you think you need. That works for airlines aswell as PPL flying. I regularily put hundreds of pounds extra into a/c I fly if I have the weight to spare. I know I have the performance to take it though.
If you know the a/c well, then you can start to refine the technique and fuel management, but if the type or particular machine is unfamiliar, then take a good reserve and use a general figure.
What's the big deal about having a totally accurate figure? So you can land and say "I used EXACTLY 55.687 ltrs aren't I clever!" what a waste of time. Flying is supposed to be fun and worrying about how much fuel you have is pretty crap.
I won't rise to the bait of questioning my instructional technique as it isn't worth bothering with, but I will say that I try to be one of the toughest instructors you'll ever meet and I don't put up with shoddy planning. If it can be demonstrated that the pilot understands fully why and what they are trying to achieve when it comes to pre-flight planning, then they are o.k in my book. Purely knowing how to look up tables and do calculations blindly with no reality check built in does not impress me in the slightest, even though my planning methods may seem at first glance to be rough and ready, they are built upon experience of what I see as really important and I've not run out of fuel yet, or failed to make it off or back onto the runway in one piece.