They look great Rigaroni!

As someone who's worked construction quite a bit and done a lot of remodeling (mostly as an electrician, but other stuff too), I can assure you that most houses are at least a little out of square. Even some modern ones are pretty far off, but usually it's the older ones that leave you scratching your head.
I put up a covered patio across the back of our house this winter (N Am winter :) ). Because the fascia was so uneven, I had to put all the joists up without cutting them, then cut the ends even. Trying to precut was sure to get messy. My new eave is almost perfectly even now though.

What you did was the perfect way to do it for custom work, unless they don't plan on leaving it there (imperfect dimensions will show up crazy elsewhere, eh?). I'm like you, and want to take out a level and get everything plumb and level. But if the more permanent surfaces around you aren't plumb or level, then guess what looks like it's wrong? Yep, even if your installation is perfectly symmetrical with itself and level and plumb, if it doesn't match the imperfection of the house, then it won't look right. So you mount a fixture on the wall measuring from an adjacent wall or the ceiling or both, and try to make up for differences by averaging out. It usually works... then sometimes it doesn't. 

I like this sort of stuff. Life isn't perfect. But we can do things to make it look better. ;)