The one jet seems to do both at the same time. The reason the jet should be placed after the intercooler is to maintain the efficentcy of it, it's the temperature difference between the air temp and charge temp, the bigger the more effeicient if you put a jet before it will effect the intercooler, the charge cooling affect is to do with the latent heat of water being 4 or 6 times bigger than petrol, so to vapourise it take way more heat to the equivelant amount of petrol, when this goes in the combustion chamber it takes the heat away as the charge is colder than it would have been, Then fuelling needs to be readjusted after it's fitted, lower, weird.
The kits come supplied with 3 jets so you can mix them about till you find the right one.
Depending on the way you want to use the system dictates how much water you want.
The main idea of water injection is for in-cylinder cooling. The jet is places as close to the intake as possible (on the plenum for the C20LET) ad the water is injected from here.
Jet size - Its trial and error I'm afraid. You dont want to much water going in. Most ppl running Phase III seem happy with the 0.7mm jets coming on at around 10psi.
With this method you arent really matcing water level with amount of air, you're matching it to bring the EGT's down down enough without over-wetting the mixture.
Some ppl (Dave Franzack) used the water injection as a way to cool the charge and the jet was tapped into the top of the standard intercooler. If you want to use this then have a look on Aquamist site under "Resources" and this will explain how jet size affects charge temperature with graphs etc etc.
Again tho, its trial and error. You can get away with a larger size jet in this case as the charge air has a longer period to mix with the water mist.
richie i have a 0.9mm jet as you know wi kicks in at aboout 0.6 bar mf2 set at 50%water at all revs is this to much water or okay and if to much water is going ij what are the consequences if i put a second jet further up what size should i use.
0.9mm is a lot but you are using the MF2 which is normally mapped to the engine to provide more water as the boost/revs rise.
TBH, the best ppl to talk to to get it right would be Aquamist themselves..... There is no real measurement or rule to use so experience counts for a LOT and I never used it on my car altho I did fit it and research the best ways for this.