dude im sure they would have said mililiters not mmeters otherwise it wouldnt make sense.
keep them moist when they are very young so the tap root stays wet.
then when they get bigger u will get the feel for how much they need by how the plants look and how dry ur soil is under the surface.
also its probs better to underwater rather than overwater if u get what i mean.
if the leaves are droopy then give um some water!