Far as I remember, as I haven't looked at the kit for quite some time, there should be stencils for the jack points on the decal sheet, and instructions as to where they go. They are inboard from the wing tip, at about a quarter span. The Spit normally used a trestle, rather than jacks, and the stencils read something like 'Trestle Here', with a black line either side of the wording.
The trestles were basically two braced vertical poles, with an interconnecting trestle bar, and the aircraft 'sat' on these. But, which U/C door have you lost? You will be able to get a replacement from Airfix, although I've been waiting about 7 weeks for a part for another kit. Or, you could make one from plastic card, using the opposite face of the other door as a pattern. There is only a slight curve on the Mk22/24 doors, so they should be fairly easy to make.