It seems likely that leading ( = line spacing) is totally dependent on the program (and/or the printer and/or the printer driver, as far as printed text is concerned), rather than being dependent on the font or on the operating system (other than the fact that the operating system has to enable the program to specify leading in the first place).
I just set up the vertical ruler in PageMaker to Picas (with 12 Points to the Pica), and placed guidelines at 15-Point intervals. I then tested a wide range of fonts -- all of which I specified as having 15 Point leading -- against the guidelines, and their leading matched the guidelines precisely. Amongst the fonts were ones that I had created in Font Creator.
In case it is of interest, here are the leading features of PageMaker and Word:
1) In PageMaker 7.0, autoleading is set by default at 120% of the font size. This percentage can be changed by the user. Also, the leading in PageMaker can be set at 0.1 Point increments for selected text, or set at 0.1 Point increments in any of the Styles.
2) In Word 2000, autoleading appears to be set by default at 115% of the font size, and is referred to in the Paragraph menu as Single line spacing. There doesn’t appear to be any way to change this autoleading percentage. Word also allows a variety of leading options under the Line spacing drop-down menu: “Single”, “1.5 lines”, “Double”, “At least”, “Exactly” and “Multiple”. If “Exactly” is specified, the leading in Word can be set at 0.01 Point increments for selected text, or set at 0.01 Point increments in any of the Styles. (However, font size can only be specified in 0.5 increments in Word, whereas it can be specified in 0.1 increments in PageMaker. Word is a strange program.)
There appears to be an exact conformance between the leading specifications in PageMaker and the leading specifications in Word, i.e. a page set in 10 Point text at 15.1 Point leading printed off identically in both PageMaker and Word as far as leading length was concerned.
Also, measurements specified on screen in PageMaker print out exactly as specified on my printer, i.e. a line specified as 200 mm in length in PageMaker prints out at exactly 200 mm in length by the ruler. If my memory is correct, the same has always been the case in PageMaker through different versions of the program installed on different operating systems and using different printers.
Consequently, it seems possible that the problems you are having might relate to the program Serif Page Plus.