Previous function in a if then statement


Hey there,

    I am trying to figure out the calc logic where the user has the ability to input a percentage in which will then be applied/reduce to the number. However i need a logic statement where the reduction is applied from previous quarter to next quarter.  For example i have 100 in Q1Fy19 and then will have 99 in Q2 Fy19 98 in Q3 Fy19 and so forth.  I know I have to use the PREVIOUS function but can't figure out the logic of how it is applied from the previous quarter to the next quarter.  


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