Hello urs.boller !
Thanks for the answer,
But I'm sill a little bit confused ... 
how do I excluded those midnight double visits ?
I understand that the summary number in the column header exludes the midnight double visits since this summary number is smaller than the sum of each row .
For example, here, I can surmise that there were 2 visits counted double at midnight.
Visits Time Spent per Visit
Day 8 x
1. 3 a
2. 4 b
3. 3 c
How do I get a total time as you explain in 1) excluding my double visits from each row ?
When I finally divide by any total of visits ( with or without the double visits) I get a wrong Time per Visit header...
Cheers !
Mario