Just maybe there is something amiss with your set up to produce that extra shift?
I set the mech to run perfectly on the smallest cog and big ring without a cable fitted. Wind the adjusters out a third on the rear cable, fit it and get it real tight. Back off the adjuster if you have pulled it over to far or wind it in to take up slack. Before you do fit the cable put some decent tension on it and triple check that the upshift button has gone as far as it can. Experience tells me that without the tension that button won't go as afr as it should and you can leave a click in which shows up in unexpected places.
I expect you have dealt with this but just a thought.
slightly better, but still not perfect. Tomorrow I'll perhaps try asking help from a mech guy, see if there's anything he can do.
I don't have an extra click now, but the mech still shifts two cogs simultaneously sometimes. I'm mentally prepared to do the following:
1. either buying a campy rear mech
2. ditch the largest cog and re-space the cassette to a six-speed one. : ) either solution can help.
The first would provide solution as the campy shifter would row with the campy mech perfectly, and the spacing of sh 7 and campy 8 is the same. The latter would be solution as well as the sh6 spacing is slightly wider than seven speed, thereby it'd tolerate that little over-pull better.
According to the tables at ctc and other cycling sites, 8speed campy with sh rear mech would be suitable for 5-6 speed cassettes, but not for seven. Thereby I still don't see how you made it possible to work, but I suppose you are more talented-experienced-have greater insight into getting bike mechs to work than me. : ) (excuse for my incompetence, I'm a beginner