Great question! We have deliberated this very issue and the discrepancies between the two manuals in our current braille transcribing course.
There is no rule as to how many numbers should be on the line before you take the division to the next line. The manual only states where a division may be made in long numbers (after a comma), and that the number must be six digits in length or longer. The reality is that you don't see numbers divided in print unless they are extremely long ... so in braille, if a number is extremely long (at least 6 digits) only divide when absolutely necessary. We recommended to our class that you not divide a number (with six digits or more) if you only had one or two digits on the line, along with a comma and the hyphen. It doesn't save space, and makes no sense to divide a number that way. In other words, if you has a number such as 3,500,000 you would not divide it 3,- 500,000 as you would not be saving much space. That is how it was explained to me so that is what I am passing along. 🙂 I hope this helps ...