Yeh merl, I'm lucky my tap water doesn't show up on my ec truncheon so its under 0.2. In coco ph can be forgiving to an extent as the medium will do some buffering, but ideally you want to keep consistently within the range of 5.8 - 6.1. Flushing isn't necessary in drain to waste coco if you are achieving 10 - 30% of the amount of water you feed as runoff. You will be flushing and replacing any excess unused salts with each feed. I personally don't think starting off in a larger pot is the best course of action (unless you are using an autoflower) but if you have to do that with coco i'd flush with water once every week or two, but you have to time it with your plants needs if it ain't too big you might end up overwatering it. With the flush i'd jsut give the pot a good drink of plain water then give it some nutrients straight after.