Medicine for small animal is so critical, and the dosage should according to the VET advice.
If you don't want to second guess your VET, why don't you seek for a second advice from an other VET?
There are a lot of vets out there who say they know chinchillas well when they know nothing about chinchillas. At vet school a vet student is given pretty much 1 page of info on chinchillas and it actually has a lot of bad info on there. I know this because we have had a teacher at a vet school in WI come to our seminars twice now and he has actually suggested 2nd guessing most vets.....I have come across vets that my 3 year old neice would know more than. Frankly if it is not an emergency I call a rancher 1st.....