152.
In a desperate attempt to try to find out what was happening to the NGs I was driving and as occupational therapy to keep me "alert" on the Hurghada and similar rotations, l took a piece of graph paper and plotted accurate fuel readings every 10 mins. (I know, sad b'std!)
I plotted at the same event, FMS fuel off the cdu-optimistic, guage totals to nearest 10kgs and finally ramp fuel minus engine totalizers at the same moment.
Assuming the pre takeoff apu burn was a constant and did not significantly affect the result, EVERY 'frame I flew showed an alarming "loss" of up to 200kgs at around the time the ctr tank pumps were selected off iaw SOPs.
Later in the sector, the "missing" fuel mysteriously recovered and despite presenting a number of graphs to the engineers, nobody could put forward a reasonable explanation.
All this was done whilst checking bowser uplift in litres against the kg uplift converted with SG at the time, an old Dan-Air precaution which has been forgotten by the newbies in flt ops management on the basis " not invented here, so we don't do that".
Apart from keeping you awake during the dogwatch, this plotting may provide an answer?
However, unless the observed imbalance/errors cause a sphincter-tightening moment or two, personally I'd use the spare mental capacity to look out of the window, or to practise the black art of weather radar tweaking or planning where I'd go in a hurry when something goes 'twang'. Or asking your (junior?) oppo in the rhs what would he/she planned to do if it all went pear-shaped whilst you were in the loo?
There again, it depends if/how soon one reaches the boredom threshold, if at all............
