definitely closer to the pump though. That allows for higher static pressure, and greater contact time, and saturation. A dedicated pump would help. The Mazzei injectors are about the best in production.

Update: I just noticed, Do you have the venturi, and ball valve transposed in that configuration ? Shouldn't flow control be on the main line to direct pressure to the Venturi valve ? Never mind me I'm just curious by nature ! ;^)