Sum and Count

I have a spreadsheet that lays out volumes from Left to Right according to the volume shipped per day.
I currently update this information weekly - and change my SUBTOTAL formula each week to be through Day "X" depending on what day I have information through......
I would like to find a formula in which I can insert the Day ("X") that I want the subtotal to go through in one cell in the sheet - and the formula would know to sum up days 1 through "X" instead of me having to change the formula each week.

Any ideas?

I have attached an example.

Perhaps?

=SUM(B4:INDEX($B$4:$M$4,C1))