If included rebased AWS from start of year then,

Amount received = 3 months AWS on previous salary + 2 months AWS on new salary with MDR + 2 months MDR
Amount received = 3/12 * prevSalary + 2/12 * (newSalary + MDR) + 2 * MDR

Copy this into wolframalpha.com, replace amountReceived prevSalary and newSalary, and the solution for x is your MDR

amountReceived = 3/12 * prevSalary + 2/12 * (newSalary + x) + 2 * x