=Sum(C:C)-Sum(C1:C5)
Sum everything then remove the sum of the values in the cells you don't want, no Volatile Offset's, Indirect's, or Array's needed.
Just for fun if you don't like that method you could also use:
=SUM($C$6:INDEX($C:$C,MATCH(9.99999999999999E+307,$C:$C))
The above formula will Sum only from C6 through the last cell in C:C where a match of a number is found. This is also non-volatile, but I believe more costly and sloppy. Just added it in case you'd prefer this anyways.
If you would like to do function like CountA for text using the last text value in a column you could use.
=COUNTIF(C6:INDEX($C:$C,MATCH(REPT("Z",255),$C:$C)),"T")
you could also use other combinations like:
=Sum($C$6:$C$65536)
or
=CountIF($C$6:$C$65536,"T")
The above would do what you ask in Excel 2003 and lower
=Sum($C$6:$C$1048576)
or
=CountIF($C$6:$C$1048576,"T")
Would both work for Excel 2007+
All above functions would simply ignore all the blank values under the last value.