How To Sum Differences of Cumulative Values/Measures?

I need a little help trying to figure out how to (What I'm going to call) summing the difference of a cumulative value/measure. 

I have the following model where I am trying to combine Forecated values with actual deliveries.

Based on this Model I display the data in the following way. 

Based on the data above the end goal is to come up with a calculation to provide the % shipped on-time which would be:

(57-4)/57 = 92.98%

I'm struggle with creating a calculation/measure that will provide the result that I'm looking for. Part of the issue (For me) is that this difference is based on 2 cummulative meaures and I can't seem to figure out the correct calculation or formulms to use.

I'm simply looking for a way to issolate that Qty of -4 from Week 4. 

Here is the DAX for all 6 columns above

Diff 2 is ultimitally giving me the correct answer (on a row by row basis) but I need to create a column that will sum/total all the negative values in order to perform the following calculation:

(57-4)/57 = 92.98%