If you feel the need to test the transit, then a good way to do it is get two set up in the middle, and get two opposite blocks level, so that you have to spin the transit around. Then take the transit down, and move it so you can shoot the two in line, without the need to spin it, the numbers should be the equal to each other again. If it passes this, then it is still calibrated and you can use it. Those things can get out of whack, even a little can equal alot over a distance.