First off, you’ll need to make a new table to do the processing. I typically call this table [Control]. In this table you’ll only have a single record. For simplicity, the primary field value of this record is just a period ("."). Back in your main table, you’ll add a Link field, point the first record to this lone record in [Control], then drag-fill that across all of the other records. Here’s what that might look like:
Whenever you add a new record to your primary table, you’ll need to also add this link, or it won’t count as part of the later calculations.
In the [Control] table, add a Rollup field:
That will give you something like this:
Make a new formula field named {Yes}, using this formula:
LEN(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(Rollup & "", "Not Yet", ""), "No", ""), "Yes", "Y"))
In short, this removes each “Not Yet” and “No”, replaces each “Yes” with “Y”, then counts the character length of the result. The three “Yes” entries become “YYY,” which is a length of 3.
Repeating this process with modified versions of that formula to make {No} and {Not Yet} fields, we have this:
One final formula field does the math you described above:
(No + {Not Yet}) / Yes
