Placement and creation of a dynamin array

I have a report that returns between 100-200 records.

Sorted by @name+date

It is only 3 data columns.

Name - allocated - date

What i want to do is create a 4th column which is the

calculated by "current allocated" - "previous allocated"

i have this working with a formula that uses PREVIOUS/NEXT

However i am unable to total/summary this formula or even

see it(see Summarize a formula which has Next/Previous)

so what i want to do is create a dynamic array for each column.

Name - allocated - date

How or where do i place the formula that holds the array data ?

thanks