I meant to add that if you don't have your pH pen yet you risk serious harm so that's good. The vast majority of problems are related to improper pH balance. It is critical. Can't say it enough. I don't think your tap water ppm is going to be an issue. Mine ranges between 650 - 850 depending on the time of year, or so it seems. That said, I don't know tons about it so read up and perhaps post in one of the other forums. I don't think it has caused me any issues if that helps.

A little more 411 on what I do with watering - starting on day 4 or 5 after they pop up I give them 1/2 dixie cup every 4 days. I dispense directly around the seedling until they have 2 or 3 sets of leaves after which I increase water. Small roots so no need to water the whole pot. Plus it is likely still holding water from the flush. Also, It is always a good idea to wet the surface of the soil with your spray before hitting it with the water. This prevents pooling that can result in uneven distribution below the surface. For those that add nutes (not you gigimarga ), it is good practice to give your plants a dose of plain water before watering in the nutes. Wetting the soil first allows more even distribution at the root zone plus you will use less nutes.