You can actually eliminate
1. from the calculation, thus reducing the uncertainty.
There's a very useful chart to visualize it,
"Application Status Dynamics" on the Charts tab. Length of each bar on it is proportional to number of applications in the spreadsheet. So in visual terms, it accounts for
"2. Number of SS apps received on each particular day", and calculating
3 as well as the estimate just means drawing a straight line that touches first dates on which fee charge is reported for each particular App Received date.
In FSW 2013 it was a straight line and then it slowed down after a few months. In our case, it was a straight line and then it sped up (as shown by the shorter line). First I thought it's just a noise, but after May 26 applications charging has started today, it was too obvious that CIC has increased their speed.
So this is basically a visualization of how the charge date estimates are calculated in the spreadsheet.