I regularly do sales run-offs where in I have an assumption of how many sales
are made in a month (say 10 per month) and each customer subscribes to my
services for another assumed figure (say 10 months). Currently, i do an
array manually, like this, then sum the total sales for the month.

Sales: Sum Sum Sum
Month 1 xxxxxxxxxxxxxxx
Month 2 xxxxxxxxxxxxxxxxx
Month 3 xxxxxxxxxxxxxxxxxxxxx

I would like to be able to "play" with the assumptions (in this case 10
sales per month with each client subscribing for 10 months) and need a
formula to fill in the array. Is there anything?