RO water, seedlings and feeding the good stuff

That is not a high number on your tap water. Just be aware of it when adding your nutrients. A lot of macronutrients are in tap water that are available to the plant so it’s really not a problem to use. You can do a lot of research into RO and spend some money on a setup you really don’t need to when it comes to growing this plant.

you don't have to set water out for 2 days.. that's a myth. It doesn't sound like your water is too hard, so you don't need filtering, but if you want to do that, it's also fine. Up to you if you think it is worth the expense and buying new filters in perpetuity etc... there's the ppm that is contributed from your fertilizer and what is coming with the water -- doesn't mean it is all plant available out of your tap. Also, the total shouldn't be high enough to cause any problems, regardless. My "city water" is 300ppm. If i had "well-water" it'd be 500+ (this area has very hard water) and that's a range you start thinking about filtration. Those hose adapter ones "boogie" brand / in-line rv filters etc can filter out 25-33% or was do they filter down to 25-33% of tap ppm? Those last 10,000 gallons, if i recall. Cheaper but not as effective, obviously.

150 ppm is fine just add fert to 300 ppm and then water if u'll need more then just add some CalMag