Using calculated columns in pivot table

I already used calculated fields to get some datas in my pivot table. They're displayed as count or sum. I want to make a new calculated fields using a Sum of working time, divide it with the number of operations. How can I use the count and the sum ? Right now, it doesn't seem to use the sum nor the count :

Here I want Sum of MBTF to be : Sum of working time/ Count of Stop but the values in Sum of MTBF seems to use the values in Stops (they are dispalyed in my original table as 6/7 numbers, but I used the function count to only get the number of occurences). I believe that's the thing causing my Sum of MTBF to show 1.68E-07 numbers ( for the first line it should be 52.001) :

How can I get to have a column displaying the right values (52.001, and others for each line) ?

Thank you in advance for your help !