Golf Spreadsheet

I'm trying to make a golf score spreadsheet. I've got all the basic information but I'm not sure how to do something. I'm making into to total scores and averages for individual golf holes.

If I play a round of golf and only play 17 holes instead of 18, how would I get the total to formulate for only 17 holes instead of 18.

I have two "totals" cells for each round submitted. One is just a total sum of strokes for example something like 52. The other is how much under par that is but that is where I'm finding problems.

I'm entering total strokes for each hole and I want the second total box to be how much under par based on how many holes I play.

So my question is, do I have to go in for each time I complete a round and figure out the total par for the holes I play (for example =C19 [which is the first total] minus 51) or can I put in a formula or conditions based on how many holes I play or data entered to figure this out?

Does that make sense? Sorry if that is confusing.

I thought of another way to phrase my problem. If I'm figuring out average and I have three rows of data and I want the average for each row.
The first row looks like this 2 5 2
The second row 3 3 3 3
The third row 3 3
For the first total average I would say something simple like =SUM(A1:A3)/3
And the second I would say =SUM(B1:B4)/4
The third =SUM(C1:C2)/2

This is where my question comes in again. Do I have to put the total number of cells entered each time or can I put in something that will see how many cells have data entered into them?

Thank you so much for the help.

Have you considered:

http://www.excelfunctions.net/ExcelAverage.html

Well, that is a simple solution for using average but that doesn't answer my question. The average part was just an example. I appreciate the consideration though. Can anybody help me with this? Thank you.

Does this help?

It does Jogier thank you. I also figured out another way to do it. Thanks again!