I need a flexible formula to find the average of the top x% of data in tables
of varying size.

To be more specific, I have daily data for many years and I need to
calculate the average if the top x% of the data. To make it more complicated,
each year is missing a different number of data points, so this formula needs
to calculate how many points to average.

I've played with Sumif/countif, percentile and Large, but can't get anything
to work. Any help would be appreciated!