Easiest way to figure this is, ratios are unimportant.
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The ratios given by manufacturers is usually the theoretical best case scenario of how much the injector can draw. As in, lowest rated flow rate (on a 2-4 gpm injector, theyāll rate it based on a 2gpm machine), no back pressure from long hoses, short soap tube, etc.
In real life, they will draw a lot less.
But @Innocentbystander is correct. Ratios donāt matter for most situations.
If the growth on the siding is turning colors and starting to come off within 5 minutes or so, your mix is good. If it sloughs off 30 seconds after it hits the siding, itās too hot - add some water. If itās not budging after 5-10 minutes of dwell, itās too weak. Thatās all you really need to know for washing in the real world.
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Youāre confusing the orifice size with it being a ratio. I think youāre confused because you originally thought the 2.1 mm size was its draw ratio. Ratios use a : between the two numbers not a period. The injector orifice size is in millimeters and usually 1.8, 2.1, 2.8, etc. The size of the injector is stamped on the side not the draw rate. Donāt even worry about the orifice size when trying to figure draw rate. Most of us step down a size which helps with a higher draw. For example, if using an 8 gpm machine youād use a 2.1 mm injector.
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I agree that when downstreaming, calculating an exact ratio is a waste of time. I also think itās pretty important for new folks to understand how calculating those ratios actually works though.
You donāt want somebody trying to use their new x-jet, doing some bad math, and having to replace a bunch of faux stone because they THOUGHT they were applying 1%
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Professor qons
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Howād you find that old pic of me? My labcoat is still white but the vest and bowtie are now royal blue. 
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My accountant would be laughing at the amount of mathematics going into this 5 gallon bucket.
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Hereās what Iāve learned, over the last year, about downstreaming. The strongest possible mix, with a 4gpm machine in theory, is 1.2 or maybe 1.5 % if you get lucky with a fresh batch of sh. So, If the house looks pretty clean and itās painted you can cut your sh with water. If it looks dirty, or has lots of organics, or it is brick/ stucko leave it full strength. That is all. Downstream ratio class over. 
If you canāt clean it with 1% you canāt get it with a downstream injector.
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Is that because of a physical limitation the downstream injector has, like a max volume that one could pull before not working?
Also are you reffering to full 12.5 sh?
Thanks
Injectors can only pull so much mix. Thatās why a 4 gpm machine can pull a stronger mix than an 8 gpm machine. Even with an 8 gpm it still pulls a strong enough mix to clean most siding. Real thick algae might take a couple applications though.
Iāve read how chem concentrations on 8 gpm machines are lower than on 4 gpm machines, but Iām not sure I understand it completely. Do they draw at the same rate, but since the chem from an 8 gpm machine is getting mixed with 2x the water, then by default the concentration is lower?
Guys have been pressure washing for decades and through trial & error have figured out the best way of doing things. I guess Iām feeling frustrated that Iām just getting into the game now and donāt have a way of reading about the trial & error done in the past that resulted in something becoming the āstandardā way of doing things, and hearing why something else doesnāt/wonāt work.
Iāve tried googling about this but I canāt find anything. What would happen if the hose barb on the downstream injector on an 8 gpm machine was replaced with one with a larger orifice so you could use a chem hose with a larger I.D.? The larger hose would hold more chem, but would the 8 gpm machine pull it? Obviously thereās a cutoff point somewhereā¦ you couldnāt run a fire hose from a chem tank to a DS injector and expect the pump to pull chem through it, but where is that cutoff point?
Here you go - this will get you started
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Yes, the strongest solution you can get with a 4 gpm machine and 12.5 sh is 1.25%. Thatās more than enough for the average house wash and just right for a really bad house. Really dirty brick and stucko may need more.
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Hereās the facts. If you have an 8gpm machine just use full 12.5% and surfactant. You wonāt even have 1% with any injector, even the fancy ones that claim stronger pull. and hopefully even then itās strong enough. Honestly to me itās a waste of time to dilute sh on your downstream mix as youāre using so little anyway. More likely the case is youāre not gonna be strong enough and youāre gonna have to either 12 V it or upgrade to a gas powered system and start cleaning Stucco, roofs and other heavy growth quickly and easily.
Not a lot of facts in all those words.
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I just installed a 1/4 metering valve on my DS line because even though my 8.5 GPM pulls 18 to 1 and my SH strength is just at or under 0.7%, some vinyl houses just have lights black dots on the siding and gutters which can be cleaned with half that strength. SH is corrosive so I try to only use what I really need.
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True, I keep forgetting about this mysterious vinyl houses everyone speaks of. For me time is worth more than $5 in sh and so I just go full strength right off the bat. How many times do you find yourself hitting something twice because it wasnāt quite strong enough diluted? Plus now here in Cali we have a draught and canāt waste water treating things twice.
Sometimes I do have to apply twice but using less chem and soap means rinsing plants and homeās less and less risk for window streaking as well. Rinsing is 80% of our time.
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Iāve been using speedx777 from espec. It really helps cut down on the rinsing time.
How does it do so?
