Commercial type of lettuce doesn't vary that much in size and hence gullies are often pre-drilled to a standard size. You have to settle for some kind of standard size somehow, as you can't change the space next season when changing varieties - or can you? Well, you can always try
In case you have a bigger variety, and it's getting narrow, you can always harvest earlier.
Actually I've ordered a new type of gully which has an open top (has removable cover). The cover is made from some Styrofoam and can in fact be changed easily. And, different spaces can hence be used. Also the gullies are much easier to clean with this type of removable top.
In commercial system you ALWAYS have several runs of differently spaced holes for different growing sizes (like shown on my pictures). There is a rotation and the seedlings grown in special nurseries will be moved to smaller spaced runs first, eventually to a larger spaced- or directly to the final, wider spaced gullies.
If running a smaller or medium scaled setup and intending to rotate production, one might even consider having one half spaced run for a pair (2) standard spaced. It is certainly worth a thought!