I want to sum a portion of a column until there is a blank cell. The number
of cells varies too much to use a bunch of nested IF functions.

For example, I'd like C100 to be the sum of cells C99 and above until there
is a blank cell. The first blank cell (working upward) may be C75 (or C65)
and there will be additional values in the cells above the blank cell.

How can I do this? Also, it would be helpful if I could count the number of
cells until the first blank cell.

I can reverse the order, that is sum the cells below a cell until reaching a
blank cell, but there will be additional values below the first blank cell.

The spreadsheet I'm working on has one day per row and the grouped rows
(days) have between 15 and 25 days in each group. The total number of groups
is over 200 so summing each individual group takes for ever (I know because I
tried that)

Thanks for your help.