Failure of the FWD pump can result in an unusable quantity of as much as 59 kg if above 80 KIAS and is 3.6 kg if below which is effectively the same as normal unusable fuel in this cell.
Failure of the AFT pump can result in as much as 71 kg in the hover reducing to 7.5 kg above 80 KIAS increasing the unusable by 4 kgs.
With a little arithmetic the maximum increase in unusable at the failure of BOTH pumps could be as much as 215 kg in the level attitude. This would be the maximum that you couldn't get to by being a little bit cunning. This of course depends on when both pumps become inactive.
If the fuel is no longer being transferred to the Supply tanks by either gravity or pumps there is 92 kgs remaining of which 86.7 is usable. Due to the shape of the tanks Engine 2 will stop after approximately 23 minutes and then engine 1 after 27 minutes. The "LOW FUEL 1" and "LOW FUEL 2" Warning indications come on at the minimum of 24kg per cell. Hence the procedure "Land within 8 minutes."
The above is based on manufacturer's fuel flow data of 65% Q at SL ISA - give or take.