Need to build a excel calculator

I want to build a simple calculator for my assistant so that way she doesn't have to take the time and calculate everything herself.

I want to build a calculator that will generate the specific day that a client will be eligible to renew.

key notes on the calculators calculation
A client will always be eligible to renew once there 45% paid in
payment types are always either daily, weekly (Only business days)

so for example if the loan was done on January 2nd for 100 payments. The estimated renewal date should be on March 6th that should equal to the 4th payment (45% paid in)