You follow the instruction on the bottle, so many drops per gallon (I think 2) for the first 48 hours. After the first 48 hours you boost the drug to full strength and measure the water for the correct level. You can then make any needed adjustments.
Make sure you test the level every day, evaporation can influence the reading.
Also dose and test your change water, this makes it easier to keep the dosage at treament strength.