Bo, this is the screw length i had with my ED80:
http://core-au.net/astro/IMGP0033.jpg
(once you screw it in, it goes a bit further in but doesn't protrude through).
This worked fine.
Also uploaded some other photos.
The complete system (ED80 with ST80 piggyback) and the ED127 with ST80, as well as the mount rings without scopes.
Didn't have a photo of the ED80 + ST80 together sadly.