Pretty sure I blew this

[U_of_I_Blue]

Situation:
1 out, R2.

Pitcher engages the rubber working from the stretch. This pitcher has a fairly slow move from the stretch to the set position. R2 gets a large preliminary lead. As soon as pitcher starts to come set, runner breaks for third. F1 does not finish coming set (hands were still a few inches apart and there had been no discernable stop) and throws to to third and retires the runner. I was so shocked at seeing this (first time i've ever seen a runner try to steal a base before the pitcher has even come set), that I didn't balk it. The longer I had to think about this though (it was last night), the more I think I should have balked him because the rules say that once he begins to come set, he must come to the set position without interruption and in one continuous motion. So, I'm pretty sure I kicked not calling this one. Am I right?

-Josh