Chicks sneezing.....terramycin dosage questions...

Does anyone know how much Terramycin I'd add to a gallon of water for chickens? It says on package 400-800 mg/gal, does anyone know how much that translates in tsp/tbsp amount per gallon?

My chicks are eating great, growing like weeds, seem to have plenty of energy, they look very very healthy. The thing is that when I go in to give them their afternoon treat some of them have been sneezing. I noticed them sneezing when I'm in there about 4 days ago. They don't have any crusts on their nostrils, their eyes all look good and they are not listless. I gave them some Sulmet for 2 days in their water. They are still sneezing. Could this be something they do to get attention when they are getting their afternoon treats? Should I redose with the Sulmet again? or try the Terramycin? Or am I worried for nothing...like I say, otherwise they seem wonderful, growing like mad and appear very active and healthy. They haven't had access to any wild birds or anything like that. They haven't even been outside their coop yet. Could it just be a cold and run it's course? Any help would be appreciated. I'm worried sick.

How much are they sneezing? Mine sneezed quite a bit when they ate. ( so pretty much always) Then I switched from mash to pellets and it stopped! I think they just had dust in their little noses!