Yeah with kalk its pretty difficult to run your pH too low. 7.9 is absolutely fine. Considering that kalk doses in equal parts, I would mostly just keep an eye on alk. Calcium is much more available so it's not going to deplete the way that alk will. Just test it every now and then. If you're keeping your alk above 7 dKH with kalk, your pH will pretty much always be in check as well. Or at least that's what I experienced with kalkwasser.