Hi there,
I've always been inserting hte ALTN RTE in the F-PLN page, with an SID, a route and the arrival as per our company OFP... I've normally got a value within a 100Kg... the difference arises when we update the INIT B page, with the actual weights from the loadsheet. As they are normally less than the planned one, the alternate fuel becomes a bit less due to the weight reduction.
As our colleague said before... the ALTN flight plan is normally computed at Cost Index 0 to allow Max Range. At least, this is what I know from our LIDO system.