Probably should have posted this a bit earlier, but couldn't find the site again to save my life.
With the issue of different manufacturers making different diameters different numbers, it makes building a leader so much more complicated.
Well, going back to when tippet was made from silk worm gut, you take the tippet number, subtract from 11, and that will give you the diameter in one-thousandths of an inch.
So if you have 7x tippet, the diameter is (11 - 7) * 0.001 = 0.004 inches.
Here is a link that explains basically what I just said, as well as provides a chart for 8x - 0x.
Fly Fishing Leader X-Designation Chart