(ET) Advice on filling batteries

[john rieffel <rieffelj union edu>]

The Crown 225AH batteries I bought for my E-15 at the beginning of the
summer have been fantastic, but a peek under the caps suggests the
water is getting a bit low.  The plates aren't exposed yet, but I can
see them a few mm under the water.  The internet is full of advice of
what *not* to do when filling batteries, but lacks specific advice for
how much to actually fill the batteries.   I don't see an obvious fill
line, but the vent holes extend below the top of the battery toward
the plates.  Should I use the bottom of the vent holes as a fill line?

regards,
jr