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.
Why do you expect a special behaviour? groff converts everything to
internal units `u'; for the `ps' device, 72000u = 1i.
Probably, I'm misunderstanding your question, so please reformulate in
case this is so.
Werner