As an aside, the receptionist may have thought she was covering their *****. Lots of vets have gotten in trouble for telling people to use medications, giving instructions, diagnosing over the phone, etc.. It's actually against the law now. Every person who calls in like that could be a potential lawsuit for them killing their pet. While you're not the type, other people are. I really don't like it, because of situations like this, but some vets are just clueless on who to trust and who to not trust.
If you keep notes of what concentration of medications you have on hand, you can always refer back to it in order to get an accurate dosage. RatGuide posts the recommended dosages, so all you need to do is have the weight of the rat and the concentration of your medications.