I think it should be done before water, add 1/3 water to double check What the stand will do with that flex. This is where you make final adjustments if needed. Then once the tank is full to make sure everything is still in order after it is filled. I don’t think you get a fully accurate level before there is water in it.
Also if there is any sort of trim on the top of the tank you may need to check if it is entirely even or that may affect your reading.
I have only set up a handful of tanks over 200g so I am no expert, but this is what I have always done and also watched much more experience people in the hobby do as well.
Lol please don't remind me about that nightmare
Just kidding, as you mention I had a relatively new tank (but 325g) some months back that cracked on the bottom (LFS's conclusion was that the welded steel stand had some construction error(s)). Luckily they were cool and replaced it for free, and using 5" bracing all around instead of 4" bracing as well as a better supported stand.When I got the new one it took me literally several days of playing around to get it what I deemed good enough (as I'm not mechanically inclined). It was really starting to mess with my head lol. I had to fill and empty the tank several times, it was a PIA but necessary. And yeah that's right Matt, good advice to measure with and without water -- and at different levels, even then it will settle a bit over time. Also yes trim or bracing that's not 100% level can throw off readings, I found it most accurate to actually measure water level at different points around the tank, using some thin metal shims where needed.
