Just to clarify,
Any amount that is in the RTE RSV box is taken account of when calculating the EXTRA fuel displayed, at least it is in our aeroplanes.
This comes in handy when (as is often the case) the FMGS ALTN fuel prediction is somewhat less than our flight planned ALTN fuel (different allowances/procedures etc.) - you can put the difference into the RTE RSV to give an ACCURATE prediction for extra fuel.
And it definitely does use current FOB minus fuel to dest+altn+rsv to work out 'extra' fuel - on long flights you can clearly see the prediction change as you change level/take short cuts etc.