There are at least 4 ways to resolve this.
I've seen quite a few comments to this effect about ADA AS, but in the distant past, peat and driftwood etc had the same issues. Some make a rather big deal about it.
One, do a massive water change, say 90% and the new water will only be influenced by 10% of the remaining water. Repeat the next day.
Now most of the water is pretty much normalized.
The substrate takes some time to extert the pH depression.
So you can measure things quickly there after.
Two: take a sample of the TANK water, allow to sit for 24-48 hours, measure the pH. do the same thing with the tap/RO etc, whatever you refill with, let sit for 24-48 hours.
Now measure their pH's and KH's. What the differences? You basically correlate a difference between the actual CO2 vs the tank water's CO2.
Add those differences to the values you measure in the tank.
Say the tap is pH 7.5
And the Tank is pH 7.0
When you target the pH, say it's 6.2 for the good CO2 ppm range, you now add enough cO2 to drop it to 5.7.
Also, measure KH as well.
Realize also, that the effect is variable through time.
So the first few hours, the tank/tap after a large water change are similar, two weeks later? A lot different.
3. pH/KH ref drop checker, check the articles and threads here for more on that.
4. pH/KH ref pH probe membrane method, again, check here.