Hi, there are many things you have to take into account about your alternate fuel and many are describe in posts before mine but in my experience I rather not change any thing on the fuel predicted page, just is very important to feed the FMGC with all info related to your alternate and is very difficult to make the FP and FMGC match because the FP is made with specific weight,rute with specific sid and star,runway,flight level, and winds so all these conditions change from the time the dispatcher do your FP and the time of your flight so... As well is important to know that the fuel in your paper flight plan is the legal so you need to land follow these fuel.
My two cents
Toby.
