You're not trying to balance fuel in this scenario. If there's more fuel in the dead side then there's obviously not a significant leak, and with 750kg on the live side even if there is a leak you'll need the other tank's fuel regardless. So that's my logic, and I'd just open the cross-feed and carry on with the approach. I wouldn't switch off fuel pumps - I'm not trying to correct the imbalance, I just want the remaining engine supplied with fuel.