About 75-80 letters per line is about 36-40em with most common fonts (excluding those in narrow and wide styles or old legacy fonts with incorrect metrics made for some South-East Asian Indic scripts or decorative designs).
Ideally all pages should avoid creating text columns narrower than 12em or larger than 50em (when it includes left or right floating elements which should be about 240px wide for images and never more than 50% of the page width between margins).
Ideally,pages should still be able to adapt themselves to wide screens (to avoid wasting it) by using columns. But if multicolumns are used, the paragraphs should not exceed about 40 lines and with colmun width set to 36em for about 75 letters and 2 columns, this means blocks of text about 6000 letters; (avoid this, you shold create subsections) ; with list of simple names (not sentences) such as toponyms, the ideal column width is about 18em, and you should avoid groupng in the same section two lists that exeed about 200 items.
Once the width of columns is determined, it remains easy to adjist the side margins to "auto" (instead of 0).
But be careful: these widths in ems are only valid for the main content (or for smaller texts sch as lists of referneces). Heading generally use bigger fonts and should continue spanning across text columns for heading levels 1 to 3 by putting them outside the multilcumn layout in a general page content of about 50em.
Don't set excessive margins and avoid builing portals with too many columns that can't fit (it is really hard to fit more than 2 columns or just 1 columns wth elements floating only on side; don't use floatting elements on both sides ! unless they are thumnailed images each with width lower than 150px).
