Yep. Simple enough and gets the point across that, like it or not, they can't all be Super-Amaze-O-Supreme Winner sessions.
This is why I like "autoregulating" and working with rep ranges instead of shooting for strict percentages.
If I'm shooting for 2-4 reps in a given set, and it's a good day, I might hit X weight. If I'm dragging that day, it might be 20 pounds less than X. If I'm on fire and it's an awesome day, it might be X + 10 pounds. In the long run, I trust it'll sort itself out because the work will still have been done up to that day's standard without necessarily needing to adjust the volume to compensate.
EDIT: For example, lately I've been training in my backyard a few days a week. Between the summer sun (tons more sweat) and the distracting mosquitos, my weights have been down a bit more than usual.
I'm still choosing to stick with it because when I do get back to just lifting just in the gym, A) I'm expecting any strength to bounce back right-quick from the more idealized situation, and B) the physical and mental benefits of lifting in my yard outweigh any "drawbacks" of lifting slightly lighter for a short period.