Add 'previous' function to equation

Hello - I’m looking to add a previous function to the below equation

“if('Is Forecast?',('Go Forward Cohort'[select:'Go Forward LTV Driver'."Borrower Retention"][BY:->'Map to Cohort Month']),
(('2.002 Cohort View - Borrower Retention Lookup')))”

I believe this formula works similar to excel where the data is either coming from the ‘Go Forward Cohort Metric’ or the ‘Cohort View Metric’ depending on the ‘Is Forecast’ boolean. I want the previous function to pull the last row of data thats a result of the if statement. 

Thanks in advance