If you go into the tortoisefriends forum website, and click on the Files section (in the panel on the left) there is file about halfway down the page called Fenbendazole calculator. Fenbendazole is another name for Panacur, and you ought to be able to use this Excel file to calculate dosage according to your tortoise's weight:
http://pets.groups.yahoo.com/group/Tortoisefriends/