ok lah, usually when u open the battery cap.... you look into
the hole you can see the water level right? shiny shiny, can like
almost see yr reflection lidat, correct? then if you notice, the
cap hole wall extends a bit down into the battery itself. just top
up the water until you see the water touching the wall and
"distorts", or you see the water like sticking to the side of the
inner hole wall. that would be the top mark, i think. no need torch
light mah 
