Fort at cap is a little complicated. Enemies have fort bypass, AFAICT, a little over 200 is enough for everything with normal fort bypass (wolves, rogues, etc.). Most things, less is fine.
Some champs have auto-fort-bypass. Nothing you can do about those other than not letting them hit you.
Some red names reduce your fort (most notably, that chipmunk in dryad raid, but quite a few things can do it). That chipmunk also has at least 100 fort bypass on higher difficulties and a large multi crit, which is a nasty combo. That will show up as a debuff in your bar.
Veng reapers (when they blow up and buff monsters) give 400% fort bypass.
Therefore, the key fort breakpoints are 100 (safe from normal mobs), ~200 (safe from sneak attacks), ~300 (safe from everything that isn't auto-critting), and *maybe* 500 (if you're in a position where you can get that, which isn't common without being a pure tank and having the reaper clickie). I'd try to run at least 120 ish on a DPS, aim for 200 if you can.
