Well let me take a stab at it:----
1/
know the fuel required to divert from your destination Airport to your nominated Alternate . This can be the number provided on the computer flight plan or the FM figures provided you have inserted all the flight plan correctly to the Altn including FL's and Wind and expected Star Rwy etc. ( note:- my company use CI 0 for their flight plan figures )
With regards to the fuel predictions page:--
Once all the programming is completed fully you can see the predicted burn off to the Altn on the top line ( fuel at A - fuel at B = burn ) , compare that to the flight plan. It is acceptable to use the lower of the two numbers but I wouldn't. The hold time at the bottom line assumes you use all the route reserve. Make the Rte Res "0" to see what the FM holding time available is. ( you can change the final reserve at Altn as well of you wish ) Then you can CLR "0" to put the default back.
Either way I just look at the Flight Plan page EFOB at arrival to see what it thinks we'll have at destination. You can also muck around with the numbers to match the CFP divert fuel as well of you are keen enough.
2/
how much fuel do you want to have on arrival at your Alternate? ( reserve, usually 30 mins )
3/
simply add 1/ and 2/ together to get 3. 
Once you have 3/ you can workout how much holding time you have. You should know the FF during holding. Then the Captain in command stuff comes in, deciding wether to hold or divert at that time.......your call.
Don't forget that the latest time to divert will be when the ECAM FOB indicates the amount or the Calculated FOB reaches that amount ( Departure FOB - Fuel used ) whichever occurs first....
Now you can apply suitable local knowledge as required to adjust up the final answer if you wish...