Assembly tip ( Y Axis rails)

When I installed my Y access rails I used a string to make sure they were straight from one end to the other. Pictures show how I did it with string.
Clamp string at each end, get 3 shims all the same thickness put one at each end and use the third to measure with. Adjust the nuts in and out until your shim just touches the string.
you should be able to get them with in .015".