Yeah a chalk line is about $5 at home cheapo, just remember to buy the actual chalk too since they don't come full lol. But yeah a chalk line is essential to rack building IMO. Do 3 lines vertical first, like 1-2" in from the front and back and then 1 more even in between those. Then you just mark each shelf as you install. Where the lines intersect is exactly where you drill.

Also, a good method to get the spacing right is to use the tubs themselves. Get the first shelf on the bottom level using shims or whatever. Once it is secured with screws then just put down some blank CDs as spacers, the tub on top of that and then the next shelf on top of the tub. Mark with the chalk line and drill your holes.