My PH swing is about 0.8-1.0
It is good that you turn your CO2 off at night, I don't know why some people keep injecting CO2 when lights are off, I've heard their reasoning but it just doesn't justify it for me...
HOBs are harder to deal with because, I'm sure you know this, they outgas. And to keep them from causing much surface disturbance, you sorta have to continuously keep the waterline near the brink right near the outflow piece. I can't do that, my water evaporates way too fast because I don't have a lid for my tank.
Your pH should swing around the same, give or take of course, also depends on your buffer/kh lvl. Well I'm hungry, be back in a bit. You shouldn't care too much about the ph drop, as long as your tank inhabitants are okay with the amount of CO2 you're injecting (if you have any) or your drop checker stays green but preferrably yellow.