Pollen on Bottom Board Tray

[cao]

Howland Blackiston says 7/10
rule, when bees have drawn 7 frames complete add 2 box, but that is using foundation.

How does that relate to 8 frame equipment? 

If it is the second box that you are adding, I would wait until they have 7 mostly drawn working on the last one.  If it is third or fourth box you can add them with less drawn.  The main reason that I do it that way is because when it is the second box, you are doubling the space that needs to be protected.  The third box, you are only adding a third more space.  The fourth box, only adding a fourth more space.