Jerusha,
I'm not a reporting expert but I do know a thing or two about UOM conversions. My suggestion is that you retrieve the base quantity and the UOM conversion factors. Perform all of the math in the base UOM and then convert to the alternate unit (if you desire to do that) as a final step for your output. If you try to do the math in the alternate UOM you will never achieve the same result that SAP gets. (Or to say it differently: "do the math the same way SAP does, and you will get the same result as SAP.)
Hope the helps some.
Dave