Originally Posted by jasdoit
2) Also, Being I've never really used a string line and really never worked with lining up post, what kind of tolerance am I dealing with or can get away with?
The kind of level that uses a garden hose can give 1/16" accuracy. Grainger publishes the accuracy of some of the levels that they sell.
Try for high accuracy, you'll always end up worse. Ideally your reference elevation should be in the center of the deck so the longest distance to any post is √2 x 7' = ~10'.
To check square, check that both deck diagonals are equal, at ~19.80'.
IMO leveling this closely is mostly for appearance.
I hope the horizontal surfaces close to your deck are level, otherwise you might have to disguise the differences, which may be visibly objectionable. The trick in this case is to make it so the eye is not drawn to this area.