And that boils down to 1.72 dia hole if I did the math right.
Yes, or 43.7mm dia hole which is 150 sq mm as I originally stated.
@Stomp, my rule of thumb for adding another box has been drummed into me by one of my many online mentors,
Every frame has fully drawn comb, or at least 90% drawn comb - no huge gaps, and
Every frame is 80% full of honey, pollen or brood, and
Every frame is completely covered with bees.
Sometimes adding another box, brood or super, too early can set the bees back. Sounds like you were pretty close.
Having a garden full of flowers is great but not all flowers have nectar and from what I understand a single bee collects one match head of honey in its life, and that's a lot of regurgitated nectar! :)
Hope this helps.