Ordered dose ÷ dose on hand × quantity. Enter the three numbers and the working is shown line by line, so you can find where your own arithmetic went a different way.
Desired over Have
Use the same unit for the ordered dose and the dose on hand — convert first if they differ.
—
Enter the three values above.
The most common error here is not arithmetic. It is an ordered dose in grams and a vial labelled in milligrams. Convert before you divide: 0.375 g is 375 mg. Doing it the other way round is how a tenfold error happens.