A lot of guys have switched to the GP stainless/kynar proportioning injectors. They can be finicky with o-rings blowing out, but using a proportioning tip in the injector is probably the simplest, most reliable way to dilute your downstream mix.
You can read all about them here:
Other options would be a pentair proportioning valve on your soap line, or a set of proportioning tips meant to fit in end of your draw tube (inside your chem tank == harder/messier to swap out)
