Average Daily Balance - Recursive Equation For Calculating Interest

I create this spreadsheet as a loan schedule using average daily balance method. (1/payment is constant, fortnightly 2/interest is 5.5% per annum)

In the interest column, at the beginning of each month ( when the day is 1) the interest will be added up from calculation of previous month daily balance.

My idea is that at interest column(let start at 1/08/2013) if (day(A49)=1, average the 30 or 31 cells above E49, 0).
I will manually make adjustment for February where 28 or 29 days applicable.