Chris,

Is this a new pool or newly refilled pool? IF your pool is 20,000 gallons, every 50 lbs of salt you add should increase the salt level 30 ppm. This will help determine if you truely have 20,000 gallons.
Regarding the test strip being in the water sample for 10 minutes. If this is a Quantab strip, where the center strip turns to a lighter color while the top horizontal strip turns dark, this is acceptable if it sits in the water sample anywhere from 4 to 10 minutes. Once the top strip turns dark, the test is good to compare. And as long as this is tested indoors, the strip will not fluctuate much, even after leaving it in the solution for more than 10 minutes.
While I agree with Grace that strips are probably quite inaccurate, I would also qualify that with, as long as the strips are maintain dry, they should be fine. If the strips are not handled properly, then you can have inaccuracies.