I am currently reading the "Beyond The Zone System" book by Phil Davis and I have a question.

He talks about subtracting the low value from the high value and then adding the difference to 5 to give the subject brightness range which is then used to determine the development time.

Example:

If the difference added to 5 = 7 then N development is needed
If the difference added to 5 = 9 then N - 2 development is needed

Question:

If on a very dull day, the difference added to 5 = 5 what would you do in development?