Okay, so maybe I don't need a PPM meter (I'm in soil).
But - should I continue to flush, or was the one time enough?
No one NEEDS a PPM meter
but it can help. I've personally never used one but to each his/her own, if it helps you grow better by all means do it up.
When you flush you're essentially washing those nutrients/salts/crap down to the bottom, this is both good and bad. It's good because it's 'cleaning' the soil but bad because the nutrients aren't there anymore for the plants. I've been using the 'make it rain' technique this current grow and really like it. When my plants are relatively dry (not bone dry) I water them with 2-3 times the size of the container, then a day or two later when the soil is still somewhat wet but getting dry I add nutrients. There are variations to this but the way I see it, this is helping correct a lot of problems I've had in the past.
Also to lower PH you can just use Apple Cider vinegar, great for the plants and most people have a bottle in the cupboards.