What IF Parameter

Forecasted Days till completion = Backlog / average completed per day

average completed per day = total completed/ number of days on project 

I created a what if parameter that changes the 'average completed per day' to adjust the forecasted days till completion

(so I have a slider where i can change the average completed per day to more of less and it will show me the forecasted days till completion based off of the number i chose on the slider) 

However, I don't know how to create a what if parameter where i can change the Forecasted Days till completion to show me how many i need to average completed per day to get the Selected Forecasted Days till completion 

(So if i change the Forecasted number of days till completion to 5, i want it to reflect how many i have to average completed per day to get that number) 

Formula for slider to adjust the average completed per day 

Backlog / [Team average completed per Day Parameter]    

Thank you in advance to any help!