Hello Blair,
I am not completely clear on what you are asking... but if you are looking to change the starting row based on the row value contained in another cell (say A1), you could try:
=SUM(INDIRECT("C"&A1&":C35"))
In this case, if A1 had the value 15, then this results in a sum of the range C15:C35.
If that is not what you are after, please describe your question with an example.
Cheers,
Sajan.
P.S. I noticed you had a duplicate of your post. One of the forum admins will need to delete the duplicate post.