plasterboard alignment with surrounds

Hi guys,

About to plasterboard a stud wall under my stairs (cloakroom WC). As you can see there is a lip of the stairway protruding past the studs..deliberately to allow plasterboard and skim. Call it poor planning or just a wonky stairs but I incorrectly assumed the protruding edgeswas same distance from the studs all the way down...of course it is not. It goes from a 23mm protrusion down to about 18.5/19mm.

Thinking it was 23 mm the whole way down I had planned to do a double layer or 9.5mm plasterboard leaving about 4mm for skim coat and was happy with that.

Now I'm stuck, two layers of 9.5 would finish flush or marginally beyond the stairs protrusion at that end leaving no room for skim coat without creating a 'step' the depth of the skim? But the alternative of 12.5mm single PB layer the whole way means a 10.5mm gap at the near end.

Whats worse or whats easier to patch up at one end? a plaster skim coat overshooting the surrounding walls by 2/3mm or having a big gap to fill with skim coat (or bonding plaster+skim?)

maybe i go get thin sheets MDF or something and the sort of shim accordingly? must be an better way

Attached images show what im talking about

Im sure there are plenty of tricks of the trade that would point one way or the other but I dont know enough about finishing walls to know the obvious answer!

 

Nope

 

you can see image ? i uploaded again

 

Pack the one end out (strip of 9.5 on the stud) likewise for immediate studs, and then feather the surface in with thicker/thinner skim.