The amount of lemon juice or vinegar needed all depends on the current ph of your tap water.
My tap water has a ph of 8.4 and it takes 1.875ml of vinegar to lower 1L of water's ph to 6.2.
I would recommend going to a petsmart or petco or something with aquariums. I was just at one today and they sell a kit that has ph up and ph down chemical already in it. It also includes ph testing strips to test the water with. All this for 10 bucks. Its way easier using that then trying to calculate how much lemon juice your tap water needs to get it down to the right ph.