The index function is rather powerful and fairly simple to use. It takes an array (in this case, A:A) and uses cell B1 to know how far down to count.
Note that in your spreadsheet, lets say the cells array that you wanted to reference began at A3 and ended at A12, you would have to change the formula to:
SUM(A3:INDEX(A3:A12,B1))
And the reference number (cell B1) could not be a number higher than 10, because that's the most amount of numbers in that array that could potentially be summed (A3:A12 is 10 rows)
You should look up coupling INDEX and MATCH. It'll change your life.
Thanks for the info!
Yeah I used to use excel all the time a couple years ago and remember using index/match instead of vlookup... but I haven't done any excel since until this morning, trying to remember all this stuff ---