So you are in the hover with 70 litres unusable in the main tank, transfer pumps off. Both supply tanks are full so you have about 20 mins flight time remaining.
Number one engine quits, number two spools up to compensate but is now using more fuel. How long have you got now?
The fuel in supply tank one is redundant (about 40 litres I think)
Only two options.
Option one land.
Option two requires a bit more cunning, anyone want to respond?
I think the clue here is, 'Number one engine quits'!
This begs the obvious question of ... If one engine has quit, why are you still wanting to remain hovering for as long as you can?