You, random Joe or Suzy Notapriest, have X amount of holy water. We'll say a pint. And you need a pint and a half of it (zombie survival pack distribution, I don't know). He says you can mix up to X-minus-a-smidgen of regular water with the holy water, and it's all holy, so you could add a cup and then have, tada, a pint and a half of holy water.
BUT that if you added a pint of holy water to a quart of regular water, you'd just have regular water.
He was not sure of the rules for starting with X amount of water, adding X-minus a smidgen (so now you have 1.9X units, say) and then wanting to add even more -- I asked if you had to wait, or stir it, or whatever, and he pointed out that he really hadn't had occasion to ask.
SO! Can you (a lay person) mix regular water with holy water to get more holy water? What are the rules?
((Edit: The internets seem to agree that my brother is wrong. Thanks, I won't use diluted holy water in any zombie kits.))
Tangentially related: Have you ever gone to other churches, not of your denomination OR perhaps going to synagogues or mosques or whatever, just for curiosity? My brother has been to about half the churches in town so far. College town, too, so there's a lot of them.