Display change in percentage week-by-week in heatmap

[elizabeth__]

I have the following calculations that give me a scrap %

And this heatmap shows that value over Date.autoCalendar.WeekYear by Platform

I have received a request to change this instead to show the difference between the percentages on a weekly basis. 

So for example, if last week our scrap % for a platform was 3.0% and this week it is 3.5%, the heatmap would display + 0.5% for the current week to show an increase. So 2019-44 in this case would show + 2.7%. I can't figure out how to calculate the change based on the previous week.