Self-assessment: How much Child Benefit to declare?

I will need to pay some of the High Income Child Benefit charge on my tax return. It is for a partial year, how much am I supposed to declare that I received/was entitled to etc
"Enter the total amount of Child Benefit you and your partner were entitled to receive for the year to 5th April 2018"

17/7/17 - 5/4/18

- CB tax calculator tells me £786.60 (which is 38 weeks)
- The self-assessment form automatically calculates £765 (37 weeks) (I can edit this, but there isn't any real reason I should be making them wrong)
- I actually received £724.5 (35 weeks) - probably because I receive it monthly.

I think it's supposed to be one of the first two, but I'm not sure why they disagree. To save manual calculations, the time period is 37 weeks and 3 days, and there were 38 mondays in this time period.

Thanks for wisdom!