Get a compact 2m retracting rule from your local Bunnings. Cheap as, and you'll use it for a myriad of things over your years in R/C.

No, alignment it doesn't have to be exact to the mm. It's desirable it is +/-2mm, but for sport flying, a difference of up to 1cm wouldn't even be noticed by most people. In a trainer, in terms of handiling characteristics once the model has been trimmed, pragmatically it'll have no adverse effect. Truth is, even a 2cm difference can be trimmed out so as not to be noticed or have undue adverse effect upon handling upon normal ops conducted with a trainer. You're not flying pattern, so +/-5mm will be close enough although with a retracting rule and moment's patience, +/-2mm is quickly and easily achievable