subtract dates and have a dropdown box that would reference to it

Hello,

I need help on how to subtract dateField2 from DateField1 in a dynamic flow table.

The table row would increase or decrease depending on how many entry the user wants to input.

I have a table with 4 columns in it. in the first column I have a date from and a date to field and a drop-down list . The drop-down list has list items which i entered in 0,0.5,1,1.5 and so forth up to 10 in increments of 0.5

What I want to do is when I change the "date work from" or DateField2 and the "date work to" or DateField1 the dropdown list would change but will also allow the user to override it as well.

How do i create this drop down?

I tried creating an invisible cell that would have DateField2-DateField1 and hoping that I can have dropdown = that and find ways to tweak from there but have not been successful.