If the font size and text box height remain constant but the number of lines changes, how can I determine the new appropriate leading?

Following up on the previous question:

You still have to also consider the Text Frame Options, First Baseline Offset. That user defined setting affects the selected text frame’s first (and last) baseline position, which could be anything. And, with the Min. setting any text frame’s first baseline position could be customized.

In all of these examples the last baseline position is changing relative to the ruler guide because of the First Baseline Offset setting:

The default Ascent:

x Height:

Leading:

Fixed:

Custom Fixed position (4.2 mm Min):

Yes. If the text frame height stays fixed, you can simply divide the available height by the number of lines to get the baseline spacing (leading). So when you go from 17 to 18 lines, the leading becomes roughly 17/18 × 16.8 ≈ 15.9 pt, which is very close to your 15.8 pt example.

Yes.

Divide the height of the text box in points with the line spacing/leading, then round down for any fractions. You’ll get your practical line count every time.

Hope this helps,

Randy