If it is too hot, turn the hot water down. If it is too cold, turn the hot water up. Other than that grab a thermometer out of the kitchen and check it. I've been using a python to do water changes for many years and adjust it based on feel. I trap the hose above the waterline with the lid, go to the sink and adjust the water, then push the thing up to start the flow to the tank. Add some dechlor and I'm done.
