The sole pupose of alk is to keep the PH from bouncing. Your PH is rock steady at an alk reading of 70, although it's at 7.8.
I'm not a fan of adding something and using something else to counteract it. I don't see the point, you're adding "stuff" to your tub and adding to it's TDS. Your PH will go to 8.4 and your alk will go to ? and then add acid to lower the PH and alk and IMO you're back to square 1.
I would only worry about the alk ONLY if you see your PH drifting around as you add product. I still believe that as you add dichlor, soak and shock with MPS (how ever often) the PH will drift down. Once it hit's 7.4 or 7.2, add a little PH up or bicarb to get it a lttle higher.
