SUMIF and Compounding formula

Hi 

I have the below dataset, what im looking to do is to say:

Rounddown(Week 02 + Week 01,0) i.e 1,022.27 -15.48 = 1,006

Then moving the forumla needs to flow each week so:

1,022.27 - 1,006 -15.48 = 0.79

Then: Week 03: rounddown(1,985.6+0.79)  = 1,986

Then again:  0.79 +1,985.6-1,986 = 0.39

going forward this needs to flow each week and be a measure

Thanks