I'm a trauma nurse and need to run these calculations all the time and fairly quickly. I have an easy way of doing it. Take the dosage that you want(500mcg) divided by the dosage that you have total in the vial 2000mcg and multiply that by the total dilution amont in the vial (2ml), this will give you what you should be drawing up in the slin pin = .50ml = 50units
now thats some solid conversions right there!! thanks for posting this!