So, coming back to this haha. Did a bit of reading between working so maybe not fully directed properly. Stumbled on this DW post...
So, the reviewer here seems to be suggesting that the diluting ratios of 1:10, 1:20 and the like, whilst fine for pump spraying as you are just adding a known quantity of product and water are probably not correct for using through a lance. Why? Well, you’re adding even more water through the pressure washer as well, thus further diluting the mix. From that review - and specific to their setup - the correct ration in the lance bottle seems to be 510ml of product to 490ml of water (assuming a 1000ml bottle). When used through the lance, this is then further diluted giving the correct dilution on the car.
To clarify, this isn’t a ‘haha, suck on that’ post. This is all pretty new to me so actually interested in others views, opinions and experiences.