Eight or nine words to the line…

I’ve been reading the book “Writing & Illuminating & Lettering” by Edward Johnston. I’m trying to learn how to write in a way that’s visually appealing. I’m only on chapter 6, section “The size of the writing”, which is about the number of words per line:

The shape, size, and margins of the page…together determine the length of the writing- line…and the size of the writing should be such as will allow a reasonable number of words to that line.

Eight or nine words to the line is a common proportion in ordinary printed books, and may be taken by the scribe as his ordinary maximum. Lines having very many words are difficult to read.

On the other hand, lines of only two or three words each are generally tiresome, though they may be allowed in special cases of fine writing…where it is less necessary to economise space or time, and the effect of an even mass is not desired. But in any case where there is an attempt to make the right-hand edge of the text approximately even, at least four or five words to the line are necessary; the scribe may therefore take four words per line as his ordinary minimum.

We may say generally, then, that an ordinary manuscript book should contain between four and eight words (or between 25 and 50 letter-spaces) to the line.