Balance point for rotisserie for series 3 OTS

[lesxjr]

I'm in the process of preparing to mount my series 3 OTS bodyshell on my rotisserie (roll over jig) and am making brackets to attach the rotisserie to the bulkhead without the engine frames in place. The brackets will allow up/down adjustment when fixing to the rotisserie to get the pivot at the balance point to allow the shell to be rotated. So that I can avoid incorporating an unnecessary amount of adjustment in the brackets, does anyone who's done this know where the balance point is, relative to any datum points on the bulkhead please. I can then also transfer that position for the boot end brackets which will bolt to the bumper mounting holes (my car is ex-USA which had the large re-inforced bumper mounts). I also realise the front brackets will need to extend forward to clear the sills.