At what exact point in time should we be giving the 10 minutes to diversion call to ATC?
(I just came up with this, so it may not be the official solution - but it should be fairly simple this way)
You first need to know the fuel you need for the diversion with all safety requirements included. This is computed like normal using the MCDU, EFB, etc.. I'll call this your &quot;bingo&quot; fuel. You just need to add the fuel required a 10 minute period for your current flight situation. Once your FOB on the upper ECAM reaches that final result, call ATC.
The easiest way to compute the fuel required for the next 10 minutes is with the fuel flow indication.
On the engine display you have the fuel flow indication, which should be nearly constant during a hold at constant speed and constant altitude.

Add all fuel flow values together
Round up to the nearest 20 kg/hr or so to be safe
Then divide the total fuel flow by 6 (10 min = 1/6 of an hour) to give you fuel required for a 10 minute hold at the current conditions.

Example:
Let's say each of the two engines shows a fuel flow of 1000 kg/hr. Add together to get 2000. Then divide 2000 kg/hr by 6 to get roughly 340 kg fuel consumption for the next 10 minutes. Add this to your bingo fuel.
( 1000 + 1000 ) / 6 = 333 -&gt; 340 ... (+ bingo fuel)
Quicker Alternative:
You could also take the higher of the two engine fuel flow values and divide by 3. Then round up:
1000 kg/hr divided by 3 = 333 kg -&gt; 340 ... (+ bingo fuel)
Note: This works the same if you have fuel flow in pound per hour. The result will be in pounds of fuel required for 10 minutes.