Find the difference in a cell value and add a formula based on that only on next three row

Hi Friends

I am trying to calculate the cost of some employees. by month.

lets assume the beginning of the year I had 4 employees and then I added one during Apr. So I need to consider recruitment cost during Apr, May, Jun. Lets say my average is 50% on the salary, I would split that between Apr, May and Jun. So the formula should find the cell with a new recruit and then add the % increase in that cell, and next couple of cells by row. Attaching an example for ease of understanding.

Cheers
Sreeram