[Groff] Point size and leading question

[Clarke Echols]

I have a question:

I haven't encountered any documentation regarding specifying
point size and leading (.ps and .vs requests) indicating how
groff behaves if I specify a fractional point size such as
12 points/13.4-point leading, or 12/14.4, for example, if I
need to make a document look better from a graphic-design and
readability point of view.

Is there anyone out there with a definitive understanding of
what groff/troff does in these cases who can enlighten me?

Thanks,

Clarke