For now, I'd keep the ingoing water to around 6.8 to 7.0 before adding the nutrients. The nutrients will drop your ph down a tad also, but once you add the nutrients, you should still be in the low-end of being within range.

I use a Milwaukee ph pen with the extra calibrating solution. When you do get one, regardless of brand, don't let the electrode dry-out. It renders the pen useless. And since I grow in soil, I've never had a need for a multi-pen.