ladder sub frame

First of all, I would like to thank every member that has ever replied to any of my threads. You all have been very helpful. With that said, I picked up my ladder bar sub frame from chassisworks yesterday and am now ready to move on to installation (cutting and welding). My big question now is what refrence points do I use to level the car. When I level the front frame rails the rear are not level. Should I use rocker panels? I can get the car to level from left to right, but front to rear not working out. I want to do this as correctly as possible without a fab shop or a jig. I do have a pretty level pad and adjustable stands to get minute adjustments. Any help would be greatly appreciated.