It's a little late now

It's a little late now as I've been using one gal of water every 2 to 3 days to water my one vanilla kush plant in a 5 gal Smart Pot and it seems to have been working ok.
But for my next 2nd grow I'd like to be more precise. How much water should I use for a 5 gal pot and a 3 gal pot. Thank you.

you'll never know for sure without weighing your pots. Do you water until runoff? Weigh your pot with bone dry soil in it and then weigh it right after you have watered it until runoff. This is your spread level of moisture. Know you'll know exactly when your plant needs a new watering/feeding!

It's not a matter of how much water by volume. Each medium and plant will take different amounts. You have to pretty much pick up the pot dry and water it only when it's close to that. Jab your finger down in there and if it's dry (Completely bone dry) water it til it's not.

I use Smartpots and I think it's really hard to overwater...they are made of breathable cloth and the water seeps out...I think the roots get more air.

I use smartpots. 1 gal, 3 gal and 10 gallon.
The 1 gallon size I use to start them then strip it off and transplant to the 3 gallon for my indoor or 10 gallon for my greenhouse grows.
The amount of water depends on the size of the plant and how much is evaporating. By placing a tray under the pot to catch the run off and disperse it throughout the bottom of the pot works good if your in a hot environment. Generally I water every other day with about 1/4 of the pot size. For example, I water three three gallon pots with 2 gallons of water every other day for plants that are about 2 feet tall and in flower. One way is to let the plant tell you when its ready to water. Start by watering real good then wait till they droop. This may take three or four days or longer but when they do you will know how long that amount of water lasts. Then water them before they droop after that feeding every other watering.