Ok I think (sometimes that is the fuck'n problem) that I've got the zero point down I have RS5036 which are 15 inches extented out of the tube, 36 inches eye to eye total, so my question is would I want my zero point to be 7.5 inches? That way I have the same up and down travel. or is there an other formula to it?

you want it so that you don't over compress or over extend your shocks. to make sure you don't do that, you'll need to flex up your rig on a ramp/forklift/ditch etc. and take some measurements. There have been many up-travel vs. down-travel debates, and it seems that about 40% up-travel is the preference, but it depends mostly on what your springs and suspension travel without shocks.