Move the paper side to side as you are lowering the nozzle. Once the nozzle touches the paper, you will feel it become slightly harder to move. This is the first hint of resistance. Then lower the nozzle again one increment past that.
Don't get caught up in being super accurate with this process. As long as the nozzle is about a papers width from the bed it is fine. Most of the time I don't even bother with the paper and just do it by eye.