Aquamist hfs-3 is what I'm using. Hands down the best system out there. Sprays off of injector duty cycle. You need to tap into the positive and negative wires off of one of your injectors. Benefit to this vs off of boost is your spray is perfectly controlled. Depending on how much fuel and throttle your car receives, the system monitors and applies the meth/water.
E92 SG M3 DCT...Powered by ESS VT2 650/DCT GTS Software/Aquamist HFS-3/ Morr /Awron/KW's/Stoptech/Recaros PP/Gintani/Catless/AA Rear section
ATCO RACEWAY: 10.65 @ 131.79MPH 1.725 60FT
(Stock ESS 625)