Seriously, there are a lot of watchers that count as accurately as they can.
Of course it's not 100%, but it's done with quite a bit of scientific rigor. After the large groups of hawks in the kettles rise, they 'peel off' and glide over to the next thermal. When they peel off at the top of one thermal, they 'stream' over...and it's in the stream that we count. Might be in one's or two's...sometimes five's.
On the 'counting twice': we see them head south. Once they've left our ridge and move onward, they count. If we see one locally, or say, dive back over the ridge, we'll wait for it to come out again before we count it.
Over a day, counting 529 accurately is easy...it's the counting 3-5,000 birds that gets tiring!
Anyway, this is the week for the broadwinged. Although the migration will continue, expect the greatest numbers anywhere this week where the weather is decent and the winds are northerly. If the winds are northerly after a storm coming down from the north, all the better.
Let me know if you get out to see this...for some reason, I find it very interesting!