There is no limit, you are most likely not supposed to play with the alpha call-up. Therefore the aircraft will warn you if the difference is greater than 7 tonnes.
But your Vapp, magenta bug, is calculated based on the FMGS VLS weight (VLS(fmgs) +5kts). The actual VLS is derived from the FAC.
This is the reason that sometimes the Vapp is less or more than 5 kts above VLS on the PFD speed scale.
My company instructs us to adjust the Vapp if Vapp is smaller than VLS +3kts on the PFD speedscale in order to achieve a +5kts margin.
Last edited by 737Jock; 3rd Sep 2012 at 15:51.