Calculating days required for an ongoing task

Eg: For a 4 day task, 50% should be completed on day2. if it takes 3 days to complete 50%, then it takes 3 days to complete the remaining work assuming the pace of work is same.
Then apply the following formula for projection calculation:
Projection (Days Required) = IIf([Physical % Complete]>0 And [Physical % Complete]<100,((100-[physical>