Meikleour - a few points to remember
a) F Reserve is sufficient to cope with ANY known normal gauge inaccuracy
b) Bowser uplift IS of course affected by bowser gauge errors and cheating swindling bowser drivers (should they exist, he added hastily) but is, of course, double-checked against your gauge readings, isn't it?
c) Do I assume you regularly adjust all your performance calculations (including landing weight when tankering) to allow for these errors? If not, why not?
d) It is a pretty silly Captain who, faced with "flow rate reductions that London apply to the TMA whenever there is significant wx" does not review his fuel diversion figure and of course, does not look at div fuel on the CFP anyway and adjust it if he/she feels necessary.
Out of interest, what is your 'comfort zone' for extra fuel? Does your company penalise you for taking it? How often have you needed it?