IT is a bit more complex due to the logarithmic scale as you mentioned, and then Kh buffering and consumption throws in some additional curves. Also if you have water with a ph of 7.0 (assuming equalized with atmospheric gases) you are adding water with a KH somewhere between .5 and 1.5dKh this is not enough to buffer well against bio-activity in the tank. So each day you do a water change and overnight the bio activity in the tank will consume Kh and the ph will most likely drop somewhat. the condition of the tank is fighting your efferts to clean things up a bit.
Without a lot more info, I hate to assume too much about your situation. so if the following thoughts don't apply, just disregard them.
Now, just in case you haven't gotten this far. test your tap water ph after it sits overnight in a shallow bowl or dish. most tap water comes out of the pipes at a different than natural Ph, and letting the gases eqalize will give you an accurate number on what you are actually dealing with.
Next thing. I don't know if the numbers you threw out were just examples, or if your Ph is actually that low, but if it is you will want to adjust it very small amunts at a time. This is a tediuos task, but the fish in the tank are acclimated to an extreme ph, and tds level, and rapid changes will cause stress.
When I have helped clean up tanks in this condition I usually do 3-4 10% water changes each day until the tank is stable with the respective tap water. I spread them out as much as I can during the day without losing sleep, so I usually have 3-4 hours between each water change. Each day it gets easier as solids levels are reduced, kh is increased, and the tank stabilizes on these issues. by doing multiple low volume changes each day, your fish have a chance to adjust to the changing conditions, but you can still change enough water each day to gain on the problems in your tank.
additional questions:
Do you have a Kh test kit?
If yes what is the tank and tap Kh level?
If no, what is the Tank and tap (after sitting out overnight) ph level
What was your previous maintenance routine (volume and frequency)?
Are you adding any Ph fixers, chemicals, etc aside from dechlorinator?
Just for some additional reference, what was the nitrate level before you started the clean-up, and what is it now?
All of these play a factor in understanding you total situation and how to best attack it. If I'm chasing wild geese, feel free to ignore me.
dave
