Something must be wrong if your tap is getting salt readings. Check to see if your hydrometer or refractometer is calibrated. When checking the SG, be as precise as possible when handling the instrument. Human error is common.
Use your 5g bucket to figure this out. Fill it with water and add conditioner. Then add 2-3 cups of salt and let it mix with a powerhead and heater for a few hours. For example, if 3 cups get you 1.004, then its easy to figure out how much is needed to get the water to 1.006 for 5g.
Its ok if you don't fill up the whole 5g bucket with brackish water because your tank does not hold 20g of water. Minus the displacement from substrate and decor and you end up with ~15g. So you may end up mixing 4g of brackish water, unless you fill the 5g bucket all the way up to the top.