This assessment pack has two pages of tests with differences of 10 & under (to get them used to the tests) and ten pages of tests with differences of 20 & under. There are 30 problems on each page. There is a spot to write the score for two minutes, and also for one minute. In my district, we give the students two minutes throughout the year, but strive to have them complete it in one minute to show automaticity.
When giving the test, I do have the students try to go in order. I do allow them to skip ones that are giving them difficulty, and have them draw a line under the spot where the sum should be. (They are allowed to go back to them if they have time.) When one minute has passed, I tell them to draw a line after the problem they just finished. The students then continue on to complete the rest of the test. This allows me to see how close they were to finishing the test in one minute.