In the end it did not help. Probably due to the height of brackets being 1-2 mm out of spec. My jig was just two pieces of MDF with carefully measured and drilled holes. Worked quite well:
Hi I have come to fitting the front brackets and was wondering is it ok that 6 of brackets have to packed out 3mm each. I have checked my measurements a 100 times and they seemed spot on. Just need reasurance before I tack them up