The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
The only thing I will note though is booming blade movement damage triggers on movement of "5 feet or more" so "exactly 5 feet" is included in this and is enough to trigger booming blade from the original effect.
It's 5 feet from the point of being struck. And at the point of being struck the target is standing on the gridline. Both straddled squares are less than 5 feet away (assuming a 5 foot square grid).
Whilst a mob must end it's turn wholly in a square, it does not have to be wholly in a square whilst it is moving.
Playing without a grid, the reaction takes place at exactly 5 ft. From that point the mob can move any distance less than 5 feet without triggering the booming blade. So they could end up 9.99999 feet from the person making the opportunity attack.
It's 5 feet from the point of being struck. And at the point of being struck the target is standing on the gridline. Both straddled squares are less than 5 feet away (assuming a 5 foot square grid).
I think you misunderstood what I was saying. I was talking about the movement damage from the original attack (made with an action), not from the reaction attack which is made when "standing on the gridline".