OK, what I think:
Before any gravity feed, you will have
FUEL L (R) TK PUMP 1 + 2 LO
PR. You select cross feed and ignition and the affected pumps OFF. And then…
WHEN TK (affected) FUEL RQRD: TK(AFFECTED)FEED.......................................GRVTY ONLY
L2 Apply GRVTY FUEL FEED procedure, (Refer to PRO-ABN-28 GRVTY FUEL FEEDING). Fuel from the affected tank may be used immediately if there is no ceiling limitation for gravity fuel feeding.
When is required? This is your decision!
Is fuel in the center tank available with operative C. T. Booster pumps? How much?
Taking into consideration the center tank fuel availability and the time above FL300, for how long will you have to maintain what level to be able to star the gravity feed procedure?
How much imbalance will develop in the cross feed configuration for that time?
What will be the final conditions when in gravity feed and the same fuel flow is demanded from both wing tank? Total usable fuel, imbalance, final altitude, position, other aircraft systems, range, etc?
Maybe you have to descend immediately to FL 150, maybe you can maintain FL370, maybe you can maintain FL370 if you cross feed for 5 min and you will develop just 200kg imbalance, maybe…who knows.
Then, standard decision making.
Only in the very unlikely situation that you have dual fuel pumps failure + both on the same tank + one engine inop + the failed pumps on the live engine side:
Just for
aircraft handling when imbalance reaches 1000 kg (and no fuel leak), you could use the bank technique (even you can fly the A320 with any imbalance).
IMHO.