Getting incremental values from cumulative figures

Hi,

I have a data set that records expected final fee and percentage complete for a number of projects, the table records one line per project per month.

Both the final fee and the percentage can go up or down as estimates are revised.

Each project has it's own unique project number.

A project will usually (but not always) have an entry for every month the project is live, even if there was no change on the project in that month.

I need to extract the incremental value change for each month, which will effectivly be:

(fee x percentage) - (fee [of previous entry for this project] x percentage [of previous entry for this project] )

It would be best if I could bring that through as part of the power query to add an additional column with the incremental change value, if that is not possible, the second best solution would be to add a column in DAX with the 3rd best solution doing as a measure.

Any help would be greatly appriciated.

Many thanks.