I found this online calculator - it has dosing instructions for a variety of drugs, given your rabbit's weight.

• http://www.morfz.com/rx/drugcalc.html
• http://homepage.mac.com/mattocks/morfz/rx/drugcalc.html (alternate location)

And here is a useful page that tells how to calculate dosages for other critters - the calculations should be worth a read, though.