To convert that to points, multiply the 0.15 by the points per line.
That will convert the measurement to points. How many points that extra fractional line is equivalent to depends on the font. If you don't want the additive extra space, you will need to reduce the 'space before' on the next paragraph to offset the extra multi-line space. So if this setting is more than single-line spacing, the extra spacing will also appear at the end of the paragraph.
Multi-line spacing is a paragraph-level setting, you can't set different values within a paragraph.
The multi-line spacing controls spacing after each line within a paragraph (including the last line of the paragraph).
