Used anything between zero and 35. I've used high tds for quite some time and some months ago gone back to 0 or close as. It's simply faster. You really need to slow down on rinsing with higher tds. Whether that's because it doesn't absorb or break down impurities as quickly I don't know. Also, some windows would never come up well with a higher tds. You can get away with higher tds, but it's simply not ideal.