How much leading?

Geoffrey Dowding lays out the norms and rules of leading in his book 'Finer points in the spacing & arrangements of type'. I will summerise the points which I think are most relevant to my project 'leading is good'.

The amount of leading is usually decided by...

The nature of the work
Books and works of reference are normally leaded in very small amount if at all. This practise has not changed since the early days of printing. Because the book has been chosen because the reader wants to read it, the designer does not have to go out of his way to induce the buyer or borrower to read. However, the unwilling reader must be persuaded to read. The copy in newspapers needs to be well leaded to compensate for the conditions in which it is often read, such as in bad lighting on swaying or jolting trains or busses.

The body-size of the type
As the body-size of the type is increased (irrespective of the x-height) it is ideal to increase the leading proportionately.

The measure
Lines which are longer than 9 or 10 words in length must be leaded proportionately in order to compensate for the extension. If the leading is not increased, the reader could read the same line twice (doubling).

The x-height of the face
Generally the larger the x-height, the larger leading may be required to achieve the same legibility.

The design of the face
"If matter has to be set solid, seriffed faces generally, with the exception of those styled 'modern', are undoubtedly easier to read than the sans serifs because the serifs help the normal horizontal movement of the eyes in reading by carrying them along the line. No such guides exist in a sans serif face and unless the lines are impeccably set and well seperated by leading there is a distinct tendancy to movement in in another direction, i.e. a vertical, or up-and-down movement. Sans serif typefaces require more leading than any other kind of type."