Diagonal Sum Product If Formula?

I am stumped on coming up with a dynamic formula that calculates a value based on an increasing number of "sums of products". Specifically, I need to calculate the total number of repeat users based on a "decaying' repeat rate. In other words, I need it to calculate = Sum[... +(d5*f5)+(d4*g4)+(d3*h3)]

or in equation form:

where
n = (1, "infinity")
and
i = {1, 2, ... (n-1)}

with
"Xn" is the calculated value of repeat users for month "n" (output/result cell)
"mi" is the number of new users (in month i)
"Nn-i" is the column 'number' of the repeat rate value (%) multiplier

Here is a screenshot of the first couple rows to illustrate:

I actually have about 70 months of data (rows) and roughly the same amount of columns representing a decaying repeat rate (70 or so) and these will be increasing as time goes on so I need this formula to be dynamic (i.e. - copy it down the column of (# of repeat users)

I am using Excel for Mac 2011.