Thanks to ATC for the comments. Unless I get more response from pilots I cannot answer for the rest!
Vertigo, I guess most people look at fuel to see what they need from commencing approach (ie leaving stack) to landing when judging if you have enough. That gives you your holding capability. To take figures of, say, reserves of 2600kg, we might 'assume' that we will burn 400 kg from the stack to t/down. We would therefore plan available hold on this figure (ie down to 3000kg). If we hold for that time and then burn 700 kg due to extended vectoring, we would have 300 kg less than reserves at t/down. That is the problem.