As David said there is no real optimal value here. It all depends on the actual content in narrow setting and your willingness to manually correct individual issues by rewriting the text, adding hyphenation points that TeX didn't find and the like.
If you set tolerance high then TeX has more possible breakpoints it considers and possibly find one that works in a problematical case. As a result you get one or a few fairly badly spaced out lines. If you set tolerance lower but use a higher emergencystretch, then Tex initially finds only "better" line breaks, i.e., those that aren't so spaced out but if that doesn't give a solution then instead of having one line badly space out you will get more (or even all) lines "fairly badly" spaced out as this emgergencystretch is then added to all lines making loosely spaced lines look better in general. What works better is a matter of taste, but imho neither is really great.
I typically set tolerance to something around 2000-3000 with a small value for additional emergencystretch and am prepared to fixed any remaining problem manually, but if you need a fully automatic solution (or nearly) then something like \sloppy or a very high value for emergencystetch (or a suitable combination of a high tolerance + a matching emergencystretch so that roughly the same set of breaks are found) is the only possibility.