Qwsa on page 1 provided the reference which I hadn't previously seen reading this thread - FCTM SI 90. I was taught it by an instructor early on in my career 5 years ago and the FMGC has had software updates since. I agree it updates based upon current gross weight and can hang my hat on it now I've seen it in writing rather than assuming or believing anonymitys on pprune. That and I have never seen a need or had a need to activate the approach that early that it updates. 1 tonne is 1 knot and it rounds up! How early are you lot seeing this on an A320 activating it?! Personally I do it when given an ATC speed on approach, when I need to select speed on approach because of my profile or at 20 miles, whichever the sooner - none of which burn a tonne or even half a tonne in the process.