Takhli, I recommend that you get yourself a small granite surface plate for that kind of work. Haha! I'm no machinist or engine builder, and I have one. And despite the picture of that beautiful Starrett that you and I would both probably love to have, I'm not suggesting you need to spend big bucks. But I do think seventy dollars would be well spent to purchase a small import.
For $67 plus shipping, here's a 12 x 9 x 2 thick example from MSC. "Shop or Toolroom Grade, 0.0001 Inch Unilateral Tolerance, Includes Certification": http://www.mscdirect.com/product/details/00150003
That would be large enough to do things like adjust thrust bearings and check main caps for flatness, etc. And according to your claims about your door, it would certified to be at least twenty-five times more accurate.
Matthew, I'm a little reluctant to go into any of the other stuff above because the storm that I ignited seems to have passed and I certainly don't want those clouds to come back. But I wanted to explain that as a fellow Z enthusiast and member of this forum community, my loyalty is to you. With that in mind, I saw a questionable tool being used by someone in your employ for something important to you, and as your ally, I thought it would be a good idea to suggest the correct tool.
It's easier to get good results using the correct equipment in good condition. It's the less common guy who can get the same good results from sub-standard equipment by making up for shortcomings with understanding, expertise, careful technique, and of course... Time.
Based on the workmanship details posted so far, I suspect Takhli actually IS one of those less common guys who has a true understanding of the concepts behind what he is doing and can get the same results from sub-standard equipment. But even with that as a possibility, I just want to make sure you are getting what you're paying for.
It's also potentially future trouble for other people trying to use the same alternative tools without having the understanding and expertise necessary to achieve the desired results. In other words, I don't think the phrase "Hey, take this over to that door over there and make sure it's flat." should become common workmanship instruction in the world of engine building. nor do I expect to find that suggestion in engine building instruction literature.
But the bottom line is that I want you to be happy with what you get, and was just trying to help achieve that end