7 1/2 is plenty for a crow, there really isn't much to them under their feathers. If you don't have decoys you would be better off setting up in trees. The thicker the better. Don't expect them to come to the call making noise, they usually will circle without making any noise. I would set up 30 yards from the caller with a good opening above you, and expect them to come tree top high. Kill them the first time they circle, if they circle more than once they usually get higher each time. They are smart and have very good eyes so if you move you will get busted, so wait until the last second to move. If there are big flocks where you hunt, 12 or more, keep calling after you shoot, I usually will switch to a different sound and if you knock some down use a crow distress sound, you will likely call some back