Yes, just before boost, how? To estimate beforehand when your going to be on boost and manually turn it on is annoying, to have it come on while the car isn't moving is pointless, to hook it up on a full throttle switch is the same as having it come on in boost, a temp sensored switch on point will see you dump all your water at the traffic lights, perhaps having it come on at 1psi might be better than my current 7psi, but at normal acceleration at only a few psi you don't need it.
If your real keen buy the Autospeed intelligent waterspray controller, it uses boost, temp, injector duty cycle among other things to control the switch on/off point.