The B's are very safe and extra is excreted. I always use tablets or capsules, and for full grown standard sized birds always just give one. For chicks or smaller pullets I will give 1/4 or 1/2. If the vitamins are just the b's then guessing is ok, a bit too much won't hurt them. If it's got others in it as well, like E, etc. those are more concerning as you can potentially give too much. I just use human B complex or Super B complex from any pharmacy, brand doesn't really matter. Thiamine and riboflavin are the ones that most commonly cause issues, but the others are important as well. It's just easier than trying to find something equivalent that is actually labeled for chickens. Vitamins are vitamins.
