RMagick Annotate - interline spacing and its relation with the font

Question

RMagick Documentation about interline_spacing mentions - "A numeric value. If positive, inserts additional space between lines. If negative, removes space between lines. The amount of space inserted or removed depends on the font."

The problem is that one can not achieve the exact text as was entered by the user. One can break lines using a small javascript and the entire text is being annotated at the server but the line spacing becomes an issue. As of now its a mere guess to set the interline spacing but as and when the font size changes the rendered text goes for a toss.

There could be an alternative to fix the line-height at the client but there doesn’t seem to be any working equation with the interline spacing.

What could be the possible implementable approaches to address the problem of interline spacing so that one can get the exact text appearance rendered out of RMagick as was entered by the user.