It might be easier to give the med undiluted, but I don’t know if you have tablets or a liquid. If you have a label to show in a picture, that might be helpful. I was just looking up dosage for someone else, and Plumbs Vet Handbook lists 25 to 50 mg per pound twice a day for 5-7 days is the usual dosage for Bactrim or SMZ-TMP.