Probably starts when diluted, but I'd call a tech at Ulano if I were you. Those guys usually love their work and are happy to help.
I got a call from Ed in Tech at Ulano, and he said that adding water to the syrup won't start the clock running . . . only once it's added to the emulsion. He did say when splitting that it was important to weigh it out carefully as it was possible to lose some of the characteristics of the emulsion if mixed out of proportion with the diazo.
He also mentioned that the effectiveness of post exposure to harden emulsion was in direct proportion to achieving full exposure to begin with, for any emulsion. In other words, post exposure wouldn't completely make up for initial underexposure.