Here is what I do for any dosing. Calculate actual water volume and not tank size. They are different due to substrate, rocks, sump, etc. It won't be perfect but it will be a bit more accurate. There are several out there if you search say on Aquarium System Volume Calculator as one example.
Ignore water changes for a minute. To calculate what our systems are using you set a baseline to see the daily use, trend, or pattern. In general get a test kit you are comfortable with, lets say ALK in this example, and test daily. Preferably on or around the same time. I use Hanna's checker for ALK because it is quick and easy and reliable for me. Test 3 times a week - say Mon, Wed, and Fri. Monday it is 8.1, Wed it is 8, Friday it is 7.9 you can do the math and see that your tank is consuming .1 ALK / day roughly. Do this for a couple weeks and you should have your pattern and initial dose.
To start the dosing pick the product, read the instructions, and start your dosing. Pretty straight forward after that. It gets a bit different based on the solution you are using. Kalkwasser vs. 2 part (ESV or BRS), etc. But this should have you a general.
TL; DR - Test a couple times a week, same time, see pattern, calculate daily use, use product calculator, set dose a bit lower, test to verify, adjust if necessary.
Hopefully smarter people will clear up or give better or clearer process but that is what I do.
Regarding after water changes or before - I mix water to match my parameters so there isn't any issue at all.
