The Mirror in the painting (NOT the Painting in the mirror!)
Maybe it's just me, but I wondered if Vermeer was somehow leaving a subtle clue to his methodology by strategically placing a mirror in the background of his painting to reflect the girl's face, as well as, the black and white diamond pattern on the floor.
This combined reflection could be used to extrapolate both Vermeer's and his device's theoretical positions within the room. The precise angle of the mirror could be determined by the shadows cast on the wall behind it.
Brilliant and, I believe, intentional.
Perhaps this is also why Tim selected this painting in particular to create his "proof of concept" painting in the style of Vermeer.
"If people like you don't learn from what happened to people like me..."-Professor Rohl