Evaportaion rates are going ot vary with the humidity in the air, the drier the air, the more water will evaporate.
Just as a reference, here in steamy muggy Florida I still lose at least a gallon a day in a 55 gallon tank, significantly more on the rare days it is not so humid.
Ohh and also.. do yourself a favor.. find the money for some kind of solution other than tap water for the makeup.. that will destroy your tank, and you will end up losing what money you have already invested in it, a good RO/DI or even distilled water (this is more expensive in the long run) will save you many, many, many hours of labor, frustration, and headache, and can make the difference between enjoying the hobby and the tank, and losing all the $$ you have already put into the tank when you get frustrated and either give up, or spend even more money and labor just trying to undo the problem the poor water caused in the fist place.
I'll step off the soap box now, sorry to rant, but good water cannot be stressed enough in its importance!