array formula question

hello all,

I'm struggling with some arrays/formulas that will make my life a lot easier if I get it right. Here is my situtation.

I need to take 1 row of cells and then multiply it and add (sumproduct) to another row of cells. Easy right. Well the more difficult part is I need to do this over and over again to a column of cells with breaks in it.

Heres a small part of my structure currently. These are the formulas I'm using for each cell down a column. This makes my life partially easier, by allowing me to copy/paste formulas.

=SUMPRODUCT(M29:Z29,$M$12:$Z$12) - (This would be cell AA29)
=SUMPRODUCT(M30:Z30,$M$12:$Z$12) - (This would be cell AA30)
=SUMPRODUCT(M33:Z33,$M$12:$Z$12) - (This would be cell AA33)
=SUMPRODUCT(M34:Z34,$M$12:$Z$12) - (This would be cell AA34)

However, what I would really like to do is make an array out of all these cells, so that when I make a modification to them (like changing the M12:Z12 to 13) the entire array updates.
The issue I have when creating the array is the function always uses the same cell input array (M29:Z29), it doesnt increment the input cell array (from 29 to 30 to 33 to 34, etc).

Any suggestions?

Thanks in advance.

From what you described I don't think you would need to change the formula. If you have this formula

=SUMPRODUCT(M29:Z29,$M$12:$Z$12)

in AA29 and copied down the column you can quite easily change the second range in AA29 only and then double click the "fill-handle" to replicate the change all the way down the column.

....or is it more complex than that?