Horizontal ones are ok, I can use distance too I guess, instead of count so that full size windows get more mullions, and windows with the parapet get less. But vertical ones I want centered to each window - whether it's a window hole with one, two, three windows...etc. Do you know what I mean?
Since creating a window itself is a bit complicated (and working now) I thought I would have an another generator for mullions. And mullions are optional too...only if a building will have classical windows.