Copyfitter is a tool that takes line length guides from Robert Bringhurst’s The Elements of Typographic Style and applies them to web typography. You type out ‘abcedfghijklmnopqrstuvwxyz’ in your chosen typeface—in the type size you’d like to use—and it’ll give you an ideal container width.
Generally speaking, if something feels right then it probably is. A solid theoretical footing is important in design, but so is intuition. Trusting our guts shouldn’t be abandoned for the sake of rationalisation. Although this isn’t to say the two are mutually exclusive. Being fairly neurotic, I like to double-check that what I’m doing holds up to some sort of rigour. When starting out a design, I begin with the typography, paying particular attention to the measure of the body copy. For this purpose the copyfitting table and techniques in Robert Bringhurst’s The Elements of Typographic Style have proved an invaluable resource.
Quite simply, the length of the lowercase alphabet of a given typeface—abcdefghijklmnopqrstuvwxyz—can be used to establish the average character count per line, for a set of predetermined line lengths.
When I tested it, the ‘ideal’ widths seemed really narrow, the highest number marked as ‘acceptable’ seemed about right (yet still quite narrow) to me; but I’m no Robert Bringhurst. Maybe I’ve been going too long all this time.