Attached. An interesting array of time calcs - you have to be careful what you use to make sure the turn of the year does not screw it up.
I used a parameter [Other Date] instead of today() in the calculations - this allows for testing and the fact you have no data for yesterday. You can use the other parameter to switch between today() or the other date.
I also used "Then [Date]" as the result of the if statements. This returns a valid list of dates that you can use as a filter (just exclude "Null). You could swap all of the [date] to [call volume] to return a number.
For averages, use the date as a filter and change the aggregation to avg...
The likely trend is interesting - your call volume is highly dependent on weekday - A Monday is 5 times higher on average than a Sunday, so prior day volume will not be a good predictor of the remainder of the month.
However, the calculation I created was: take the volume for the day prior, times by the number of days left in the month from today, and add whatever days in the month we do have a volume for.
You may want to think about using an average for the month so far, but even that will be skewed early in the month...
The only other slightly confusing issue is that Tableau's week starts Sunday - yours starts Monday, so I had to account for that. Please check all work, especially behavior around year change and Sunday/Mondays....
CallVolumeCalculations.twbx.zip 185.6 KB