I will second the suggestion to use only DISTILLED WATER, though that is NOT what is in the battery (That is dilute sulfuric acid) it is the water part that gets "lost" and thus you replace only Distilled water.. NOT tap but DISTILLED.
As to the level.. I like just below the bottom of the tube... There are assorted reasons for this.. but 1/8 inch is good,, Just slightly below the bottom of the tube.. However if you find you put it up to the bottom.. DO NOT REMOVE ANY.
Home is where I park it!