What I tend to do is look at what the FMGC is telling me what it think it will burn going to the alternate, then look at what the difference is on the flight plan since that takes into consideration the go-around and flight plan route to the alternate. I then put the difference into the route reserve instead of it having the 5% contingency. In the extra fuel section that will then show you the extra fuel you have including your contingency fuel.
I always round up and never down. When I do fuel check how much extra fuel the flight plan tells me is the same as the extra fuel the FMGC is also telling me.