My tolerance to dope is pretty concrete: 200mgs IV or 300mgs smoked/snorted, so i never have a huge risk of taking too much, since the product i get is very consistent. However, I do get migraines sometimes when I do too much in a day.
With meth, my body rejects even the smallest amount without some GABA action going on simultaneously for some reason, so it's pretty easy to take that too far, like how I had a long, dragged out comedown of which I am still somewhat in the throes of from snorting maybe 3 or four points (in two lines, but still).
With benzos, I never take a whole lot, I like to mix small amounts of a lot of downers (dangerous, i know) and 5-10mg valium is the most i take unless its the only drug i have, in which case it can go up to 20mg.
With alcohol, it's usually time to stop if i feel the need to drink compulsively even though I'm hammered.