First off, welcome to BYC, glad to have you here asking questions.
Have you read any of the Fermented feed thread yet? If not, I would recommend popping in there, LOTS of good info there to check out.
https://www.backyardchickens.com/threads/fermented-feeds-anyone-using-them.645057/
As for your specific questions, I'm not sure how much to start with, but it doesn't really go bad, it just gets deeper fermented as it ages, so start with something that sounds reasonable and then go from there.
If they don't finish it, it won't go bad, but it will dry out and be harder for them to eat since it will cake to the dish, so, after a while (a few hours, before the next feeding, etc) just scrape the stuff out of the dish and if they haven't pooped in it or otherwise made it gross, just dump it back in the bucket of FF and give it a stir.
A lot of people start their ferment off as a peanut butter type consistency. If it is too moist, just add more feed without adding water. You will get a feel for how much water your specific feed will absorb. You certainly CAN make it wetter and strain it out if you want, but that's just more work and mess to deal with, so it's probably easier to just start with a thicker mix to begin with.
One thing I have seen about feeding chicks FF is that if they can get IN the dish, they will, things like matted down feathers happen frequently and with wetter mixes, chicks have drowned in the feed. That is another reason to keep it on the dryer side and feed it in one of the chick feeders that they stick their head in until they get big enough you don't have to worry about them trying to go swimming.
I'm going to tag a few people that are currently active on the big fermented feed thread so they can come over here and chime in as well.
@Beer can @Gramma Chick @lazy gardener