Bex calculate Difference % between two dates for the same key figure

Hello Experts,

My sincere apology if this question has been asked quite a few times. I just could't find it anywhere. I have a key figure 'Sales Revenue' which is being displayed separately for the different fiscal year week. Lets say there are two fiscal year week 42.2016 & 42.2015. Now I want to calculate the diff % which will be like this (sales Rev w.r.t 42.2015 - sales Rev w.r.t 42.2016 ) / ( sales Rev @ 42.2015 ) * 100. However I don't know how to achive this in Bex since the sales Rev w.r.t 42.2015 and sales Rev w.r.t 42.2016 are the same key field.

I hope my question is clear. Please let me know if you want me to explain it more.

Best Regards