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Is it totally ok for a otf-font to have a different UPM than 1000, if the ascender and descender adds up to that other value?
Yes. I would highly recommend reading through, Karsten Luecke's guide to font metrics (fair warning, depending on your browser, that link will trigger an automatic download of a PDF). There are also numerous discussions elsewhere on Typophile, but the one about UPM and Vertical Font Metrics should have the level of detail you need and cover most questions.