Russ mentioned to you on facebook to keep it about 30:1 I would not apply it stronger than that.
Are you saying that you are using a 2gpm pressure washer to downstream? I am confused with your sentence.
I would keep it simple and downstream with the 8gpm machine and rinse with the 8gpm machine. You might need to do some brushing on the first wash if they have not been maintained regularly.
You need to do a test and see what ratio your injector is pulling, don't rely on what the manufacturer or sales rep tells you as there are many variables that will affect the ratio.
Get a 2 quart or larger measuring pitcher and fill it then with your injector rigged up, injector hose in the picture full of water and soap nozzle on the end of the wand, pull the trigger and start a stop watch or look at your watch and count out a minute and then let go of the trigger.
See how low the water went and subtract that from full and let us know how much water it pulled so we can help you with the math so you will know your ratio.
After that we can help you determine the best way to keep your ratio at about 30:1 so you don't apply the Panel Brite too strong on the vehicles.
Let us know the amount of water you end up with when doing the test so we can help you.