Yes, 300 days is a nice slow taper.
I will be happy to help you sort out a formula.
Right now, you are at .465 mg of your drug, correct?
If so, you can continue with that taper and switch to the water method as follows:
Each night, put 0.5 mg tablet into 300 mL of water.
Then, to get to 0.465 mg, remove 21 mL of water.
Now your mixture has exactly 0.465 mg of drug.
Do this each night but then to taper, each night remove an additional 1 mL before drinking it.
Does that help?