count a range that starts depending on cell value and ends depending on other rows

I have a spread sheet laid out with the months across row 12. There is 4 years (eventually I will put more years as we reach them) worth of data and two columns between each year that are unnecessary for the formula I am working on. From Row 12-150 Column N-BO is financial amounts per month. I am trying to develop a formula that allows me to average out the overall amount the customer has paid divided by overall months starting at the inception of the customer to the current month. I probably should have it laid out a lot better with some of the other functions I don't know how to use but for now I am just using this for personal knowledge. The ending month is just noted as the last column with any data in it.

Thank you.

Re: count a range that starts depending on cell value and ends depending on other row

In the attachment is a part of the actual file with the data changed. L13 is the current cell I am working on and will fill down from there.

Thank you,
cya

Re: count a range that starts depending on cell value and ends depending on other row

I'll try to make things more clear.

I was thinking there might be a way to have a formula where the value of a column is <1 that would show that it is past the current month. That would do for the end part. I just don't know how to find the starting point of the customers first month and then count all the months between that month and the present, divide it by that number and put it all in a formula. In other words a customer started in 6 months after our company started they paid for 2 months didn't pay for 1 month then paid for 3 and so on. I want to add all that information and divide by how many months they have been with us and keep it current so that I know how valuable of a customer they are.

I hope this helps and doesn't just confuse things more.

Thank you for your help.