edit- Ninja'd by Hoppy because I had to stop typing mid-post to scold my freaking cats
Whoa, first things first, let's talk about the drop checker... the solution that you put inside of the drop checker should be certified kH 4. Not lower than, not higher than. You should NEVER be using aquarium water inside of it. That's some eBay seller myth that somehow escaped the confines of eBay. If you don't have a certified kH of 4, you are not getting accurate readings.
Now, that being said, the kH of your tank water will have no effect on the kH of your drop checker solution. The two are unrelated.
I highly advise against adding any baking soda, or anything for that matter, to raise your pH. As far as hardness, if you are not happy with having softer water (not sure why, but to each his own!), use a product designed for increasing hardness in aquaria, like Seachem Equilibrium for instance. But don't start fighting the pH of your tank, because you are going to make this tank into a frustrating hassle for yourself!
A pH of 6.45 is good
, so you are fine. And yes, I recommend upping your CO2 to about 1.0-1.5 bubbles per second to start with. You may need to adjust that as necessary depending on your diffusion method.
LOL, I can tell that you are coming fresh from the reef world.
But in planted tanks, slightly acidic, softer water is a GOOD thing.