Calculating through empty cells (nested ISBLANK)

For whomever is interested, I realized there are two more cases which weren't considered:

1. if a cell is blank and you fill it with 0 then if you do AVG on the column it will then take the row and add it to the denominator of the AVG. You may want to avoid this becuase you don't want to count projects which did not (naturally) made it to or through that Stage. E.g. if a project is still in Stage 1 you want to ignore it if you do an AVG on Stage 2 but, if the project skipped Stage 2 and jumped to Stage 3 then you want to take the 0 value of Stage 2
2. never underestimate the user's capability to screw data. More often than not, I found the situation where the project skipped a Stage but later on reverted to the skipped Stage. YOu will end up with a negative number of the days that the project spent in the skipped Stage. I mitigated this by creating a new column (🙄) and doing something simple like IF [Column] <0, 0, [Column]

Hope it makes sense. 

The end solution is ugly af but it works 🙂