Originally Posted by JimAde
1) I believe the CON modifier is added after dividing. So if an animal has a 14 Con and 1/2 HD, it would have 2.25 (half the average for 1d8) +2 or 4 hit points. Honestly, though, at that level I just give them a couple of hit points and don't worry about it.
Hope that helps.
Originally Posted by werk
1. Can't find anything concrete for you, but I add (or subtract) the con bonus after the fraction is resolved. I usually give max HP for creatures with less than 1HD and make the HD type the numerator. So a bat has 1/4 d8 HD = 8/4 = 2HD. I do use elite array on most everything, so if you used the half you would get 4/4 = 1HP which is what's in the MM.