You really do need a culture for the RI. For mouth rot, I don't know if Baytril is the gold standard or one of many antibiotics for mouth rot. If Baytril is not a for sure cure, get a culture.
But to answer your question look at the Baytril labeling to see how many mg per ml there are. And figure out from there how many ml's are needed since you know that you need 125 mg. Clear as mud, right?
And an Amen to what Sully told you about cc and ml being the same.