Oh fun.
This is actually very informative about hair loss and it states that "The average human head (an average scalp measures approximately 120 square inches or 770 cm)..." Now that is most certainly an average since heads are varying sizes as well as heads in comparison to ACTUAL scalp.
Scalp, by definition, takes into account the varying sizes throughout, since the hairline is not a square.
Sorry but, even by measuring cross-width/length it would not equate to the square inches of the scalp because the hairline varies the width dramatically from inch to inch.
http://www.newworldencyclopedia.org/entry/Hair
Since everyone is different and we are talking about averages, not accuracies, lets just say for the sake of sanity that we are all correct.
Now, all that sexy mathematics aside, your hair looks fine Kevin. Give it a lot more time before congoing though because if your scalp is really covered with 1"^2 sections then congoing now will make your dreads super huge and possibly a pain in your ass to care for. It's much easier to go bigger than to go smaller.
Good night loves.