Thanks...I will digest all of this nitrogen information.

I've looked everywhere on the net I could find that has photos of lawn
diseases. None of the pictures look like what I have. Mine isn't
patchy or blotchy, and the green blades themselves never look like
anything is "attacking" them...they seem healthy and green.

I've already done the Scotts pre-emergent (Halts with Turf Builder).

I used two 40 pound bags, which are supposed to cover 15,000 sq ft
each. I ran out with only the small back corner undone.
I used a Scotts Brand spreader set to their recommended settings.

So, I can't be sure that this MEANS my lawn is a little over 30,000 sq
ft....I will figure out a way to measure and calibrate as you
mentioned.