Markus in Switzerland told me about this interesting site. Parts of it are in German (which I never learned to read, but can using Google translate).
Daniel has made an interesting spreadsheet for designing axial alternators. You can download it here. http://www.nadaparasiempre.com/hobbies/wind-energy/generator/index.html It allows you to design for a particular cut-in rpm accurately. But take care, because it does not calculate the rpm and the tip speed ratio when the wind gets a bit stronger. It is likely that your rpm will be too low and the turbine will stall, so the performance will not be s predicted in the spreadsheet.
German discussion forum here http://www.kleinwindanlagen.de/Forum/cf3/