"I'm working under the assumption that urine is 10% N (I've read it's anywhere from 8% - 12%, so we'll go with the average of the two). "

Where have you found this number? Since urea is 46% N, you're saying that urine is about 1/3 urea. That seems unlikely.

"I'm also working under the assumption that urine weighs the same as water, roughly 16oz equals 1 lb. "

Umm. 16 oz of anything is 1 lb. If you mean 16 fl oz, that's a different story. Metrics use water, so I converted 16 fl oz to ml (same as g) and converted that to lbs. 16 fl oz of water is still pretty close to 1 lb (1.04317556 lb). Urine is denser than water, so 16 fl oz of urine would probably be a little heavier still.

If your assumptions on the nitrogen content of urine are correct, your calculations of amount of urine to use are probably close, but not quite right. But I am very doubtful that urine is 10% N.

For a 250sqft lawn Go out and buy a 50lb bag of SBM for less than $20.00 and that should last you 2 years if you apply it @ 5-lbs per 250sqft.

You can get coffee grounds for free at Starsucks.

Wikipedia has urine as 95%, the remainder having urea as a part of it, so 10% may even be high if that's accurate info. I looked it up because my first thought was more that there's got to be a lot of variability in that percentage anyway.

Thanks for the replies guys, I bought some SBM and will be using that along with twice yearly compost dressing

Why not a compost tea?