FMC gives balanced field speeds. There should be little difference, 1kt, between the VR & V2 FMC & Perf tables. If so then there is a problem. 1kt = 1000kgs (+/-). V1 can vary for a multitude of reasons. On long runways it is common for it to equal VR via both calculations. The +/- 1kt is the gross error check. If within that boundary then use FMC speeds and manually enter V1 is outside the boundary. Simple. Switching between DRY/WET will also only change V1in FMC.