For pure interest: with fov_desired 90 & 1920x1080, trying out a custom map that looks like this, and trying out setang values with until the center line was shifted exactly 1px (had to use max antialiasing settings to compare better), I got an angular movement of ~0.062 giving me the best results. Stabby/wo1fwood's ~0.0596 degrees (from 2.71286834) and HOI's ~0.077 were both demonstrably off when I tried it. This would imply that the "pixel perfect" sens should be ~2.818.
Setsul is entirely right that it's nearly meaningless and a very poor way to pick your sens though. You will notice every other factor that goes into a sens (whether it's too high or too low for your aiming, whether it malfunctions your mouse sensor) and every other factor that causes stuttering / inconsistency (monitor hz, framerate, antialiasing settings) long before you notice your sens being slightly mismatched with the pixels directly in the center of your screen. You can easily play with a higher value than that and never notice anything wrong, because we're talking about such a specific and minor criteria that 99% of players would not notice if you pointed it out to them.
stabbyThere has to be a definitive point at which "anything" becomes "stupid".
That point is "as long as it's not bothering you." If a sens works for you, it's not stupid. If a sens doesn't work for you for any reason whatsoever, it's stupid.
You could also decide that you want no pixel skipping anywhere on your screen (completely reasonable, arguably more valuable anyway), and require a ~0.03 degree angular movement, and now ~1.36 is your max sens. Maybe you want granularity of half a pixel on the edge of your screen (fair, definitely an improvement), and with a ~0.015 degree movement ~0.68 is your max sens. You can come up with whatever cutoff for "definitively acceptable" you want, and it's all nonsense because no one actually notices this shit ingame.