That "caught it in about 10 minutes" part is the real story here. Dude saved your whole run before it even started. Rich black at 60/40/40/100 is way too much ink for a card stock, and offsetting on the back of a 500 run would've been a total loss. I usually run 40/30/30/100 too, or just 100K with a touch of cyan if the shop asks for it.

Learned this the hard way on a deck job last summer and it totally changed how I set up my laser. I used to just plop the level on whatever was close and trust it, which worked fine until a floor joist gave a little under my knee and threw off a whole run of railing posts. Now I clamp it to something solid like a wall stud or a post, never a bare joist or a staging board. The flex is sneaky too because you don't feel it move, you just watch the line creep a hair while you lean in. Re-measuring four times is the worst part, you start doubting your tape instead of the setup.