I know this thread hasn't been active for a bit but just a couple of points to clarify on this.
If you do have 1 stage of slats extended when you get double failure and you decide to fly the approach as per the QRH summary then: You ll roughly be 200kts depending on weight anyway and need to be in manual thrust. Gear down then F2 F3 and decelerate to VAPP. As you have some slat VREF +10 should be bugged as per the status and this relate to just above VLS once fully configured.
In terms of ldg district cal. G+B in QRH relates to no slats on landing and VREF +25. In reality with one stage slat looking at in flight slat/flap performance the ldg dist takes into account the increase in approach speed but not the loss of ground spoilers 5 3 and 1. Surely then your ldg dist is going to be increased over a purely vref +10 ldg distance?
Last if slats were at zero so classic G+B failure from clean would you not config athr off then F1 (I know it doesn't put any slat out but at least SRS avail in G/A mode) 200kts gear down F2 F3 then VAPP.
Any help much appreciated.