multi criteria formula with pro rata

Hello

i am going to do my best to explain how i may require your help building a formula.
on the attached document, you will see a very simple calculation for accrual formula that add and withdraw amount from a month output (colomn c)at each of every three months (march, june, sept, dec)
this document attached works when its a jan2012 thru dec 2012 12 month basis.
but what if i use the same numbers on row C and now use from may 2012 thru april 2013? how can i have a formula that will give me the exact same total?
can u please help me ?? i am desperate, i m actually beyond desperation