In the illustration, the first two text blocks show text in two different point sizes but both column lengths are far too short for comfortable reading (the outer edge of the optimum area of the second text block actually extends beyond the edge of the graphic).
The third example shows a line length that is too long for the selected font size.
In the fourth example the text falls within the ideal line length range based on the two formulas. Enlarging or reducing the font size only slightly would probably still yield acceptable results.
The results of applying the two line length formulas are a suggestion only. A little less or a little more in either direction is probably not going to adversely affect the appearance or readability of your text. But text that falls too far outside the optimum range can make your text and layout ugly and too difficult to read.
The line length rule applies primarily to body copy. Headlines, subheadings, and other small bits of text have more flexibility. Almost any reasonable line length will work in a design if combined with the right size font. The longer the line, the larger the font can be. The shorter the line, the smaller the font can be.