How "late in the day" can you do this?
Generally the V1, Vr speeds etc are calculated just before the doors are closed.
So just to recap,
in general the process is:
1. (-24 hours) Online checkin opens.
2. (-2-8 hours) Ops make a 'best guess' at passenger/baggage/catering load based on
booked figures. A flight plan is produced, with these estimates, and given to the crew.
3. (-2 hours) Airport checkin opens.
4. (-30mins) Checkin closure. The final(ish) ZFW is known, and the fuel figures are finalised by the crew and passed to the dispatcher. A loadsheet can be produced now if needs be.
5. (-10mins) All pax boarded, minus any no shows. If a loadsheet has been produced at checkin closure this can be LMCed to reflect any changes (i.e. the no show pax). However it is much easier to produce a loadsheet now, as the actual final load is known.
6. (-10mins) The crew input the final figures into the FMC, this gives the appropriate V1, Vr speeds etc, alternatively they can be looked up on charts. These are used for the take off.
7. (-5mins) Close doors
8. (dep time) Wait 10mins for a tug to show up
9. (+10mins) A/c pushes.
This obviously varies between airlines and airports, but in general its how it happens!
So the V1, Vr speeds are calculated at the last minute. There would have to be a fairly major change in weight to affect the fuel requirement, so this doesn't really change (certainly on the shorter domestic/European routes I deal with).
Hope that's some help!