Summarization for Custom data item-Please help

I am preparing a schedule with 3 calendar id
1-No Friday & Saturday
2-No Thursday, Friday & Saturday
3-7 days work week

Each of these calendars are being used alternately per activity

My problem is, I need to show the total durations base on calendar 3.

I made a custom data item, titled it DUR.
I used GLOBAL CHANGE to create formula as follows:
If Cal = 1, or Cal=2, or Cal=3
Then DUR=EF-ES
I got the result I want

What lacking in this is, it is adding up the total duration per activity.Instead of 240 days, the total is 768 days.

I look at tools=>option>summarizing but none
I also look at autocost but nothing I found.

Anybody?

Thanks & Regards in advance.

RANTE
Doha, Qatar