this is an example how to do it..

let's say you want the rear to drop about 3mm (or whatever)...
- open the sliding door, put a bottle jack under the rearmost of the door
(your left if you look at the door from the outside, raise jack about to
touch the bottom of the door with a gap of 3mm.
- loosen the bolts and let the door going drop till it touches the jack.
Tighten the bolts and remove the jack.

if it looks OK, you're done, if not repeat.

to raise the door
- open the door, put the jack flush with the bottom of the door, loosen the
bolts, raise the jack a bit, tighten the bolts, remove the jack.