For each year:

1. Sum the word counts.
2. Divide the sum by 301, the number of days in 43 weeks.
   This provides the target average lesson length for that year.
3. Divide the number of words in each book, by the target average lesson length.
   This provides the preliminary figure of the number of days for each book.
4. Round those quotients to the nearest integer, and adjust the target average lesson length up or down until the total number of days is 301.

As of 09/06/20, this has been finished as to Year 1, Year 2 and Lent, and is in progress as to Year 3. The current status of the work appears in the file, "Allotment of days to books," under "Resources," here.