At the top, you mention per the KH kit that your KH level was 7. Below you restate your KH=12. A bit confusing.
I'll assume the 7 is accurate. Any KH level above 3, should be just fine. I have a KH of 7.5 (at drop 7, the liquid turns green, at drop 8, it transitions to full yellow, so I guestimate it at 7.5), and I've been happy with it. My target pH is 6.7-6.8 because I'm targetting a CO2 level > 30 ppm. So, I wouldn't do anything about your KH, but I would try to up the CO2.