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.
Rusty Trichome Reviewed by Rusty Trichome on . ph run-off @ 5.0 What is your experience level? novice Your Equipment: .1) Type and wattage of lights? Dual core horti 1000w HPS .2) Distance from tops? 30? .3) Reflector type? Super sun 2 .4) Is there a consistent fresh air supply? Yes .5) Do you have an exhaust fan and a circulation fan? 440 cfm ecoplus supreme inline and 2 18? oscillating fans .6) What are the bulb wattages, Kelvin ratings, and schedule? 1000w HPS 12/12 Your medium: Rating: 5