Try this over in the mechanics forum - they may know something I don't. If you have a triple, 11-28 should suffice. Have you played with Sheldon's gear inch calculator?
No, although I'm familar with the site. I must confess I don't understand the concept behind gear inches. I should do a search on that to get a better understanding of that. OTOH, I may be able to find an expplanition in my Zinn repair guide.