I too highly recommend the dosage calculations made incredibly easy book by Springhouse publishing.
It walks you through all calculations and gives practice questions at the end of each section.
If you dont want to spend that much, I recommend getting books from
Its affiliated with Ebay-and its a very good site. You can find most of your nursing books for half price, if you dont mind them a little used, although most are fairly new.
I was always the worst at calculations, and now I am pretty good at them, and this books helps!
Good Luck! If you have specific questions post them, maybe I can help