The basic rule of thumb is to start with type size plus 2 additional points. For example, 10 pt type might start with a leading value of 12, written 10/12. 9 pt type might be set 9/11. But it's just a starting point: here are so many variables to consider. For example,
- If you want a more open look, and an extra point or two.
- If the typeface has a large x-height, add more.
- If the type is set in All Caps, add less.
What you want to get away from as soon as possible is using auto-leading: it multiplies the type size by 120% which works well at 10 pts (i.e., 10/12) but very poorly as the type size increases (i.e., 30/36 or 72/86.4).
Here are some resources with much more detailed examples that you might find helpful:
In addition to what Barb has said, which describes "fixed leading," you can also set up "auto-leading," which is the default condition in InDesign, as a percentage of the type size in the Justification section of your paragraph style definition. The default value is 120% of the type size, the same as in Word. You will recognize auto-leading in the panel fields as a value in parentheses.
If you select text to which fixed leading is applied and change only the type size, the leading value will not change. It will change if set to auto-leading. The largest leading specified for any character in a line will be used for the entire line, and the choice of fixed or auto will also affect what happens with inline anchored objects.
Auto-leading need not be 120%, that's just the default value and you can set up anything you want. There are reasons to use either method.
Je vous remercie pour vos réponses précises et complètes. Elles m'ont été très utiles.
Je vous en prie.