Being fixed tripod you'd be wanting to do mostly very wide (maximum open) stuff, but concentrating on nice framing of forground objects.
For startrails the ISO isn't important, I'd go for about ISO 200, and exposure times of about 30 minutes to give a nice length trail. You don't want too much ISO or it'll just all be noise, and you don't need high ISO because you're after the trail effect that'll show no problem, not faint nebulas.
For non-trailing shots, I'd go for 800 ISO and maximum of about 30 seconds. I find 30 seconds at 17mm gives minimal trailing while longer it starts to get anoying. Similarly zooming in more pronounces the trails more, so I'd keep to the 25mm or wider kind of range.
I'd set the f-stop to either full open or just a stop or 2 down from full open to get a bit better quality shot (but less light so there's a trade-off).
I'm not good with formula's so can't help with anything specific, but that's from my own practical experience.
Roger.