RO water is kind of all over the map so it may be tough to know what's in it. Some people treat RO as if it were distilled but distilled is all-zeroes-all-the-time so at least you would know where you were starting. I used to pic up "RO" water from the grocery store (in a big dispensing machine) and I had my doubts so I sent a sample to Ward Labs and found out that it had all sorts of stuff in it. The water for a Czech Lager should be soft, for sure. You could try to get to 40ppm of Ca so that you can keep things soft and just add some CaCl to get to 40ppm. I always hear that nothing is going to happen at 50ppm that wouldn't happen at 40ppm. Magnesium shouldn't be an issue because malt has an adequate amount, maybe add a smidge of kosher salt to get your Na up off the ground. Soft water mutes hops so you might have to use more hops than you would ordinarily think. For me, I typically use my source water (which is modest on most ions) and then use some percentage of distilled and add back just enough CaCl to get to about 40ppm of Ca and call it good. Good luck.