here's another mini-chem lecture:
when you synthesize a substance, you must first write an equation for the reactions you will perform. the ratio in which the various reagents combine can then be used to predict how much of each reagent you will need, using the number of moles required for the reaction and the molecular weight of the substance.
you then can calculate the "theoretical yield" of the reaction by predicting how many moles a perfect reaction would give, and muliplying by the molecular weight of the product.
when you perform the reaction, for hundreds of different reasons, the reaction will not go perfectly. afterwards, you weigh your product and divide the "actual yield" by the "theoretical yield" to get a "percent yield". 11 grams of product divided by 15 grams theoretical yield = 73% yield. it is never expected that you will get more than about 90% yield.
i shall assume that you are not aware of the concept of an enantiomer, or you would not be attempting to extract L-methamphetamine from Vicks inhalers.
what really troubles me, though, is that it sounds like you used three hundred inhalers. sketchy.
anyway, this kind of question isn't really appopriate here - i thought i'd just let you know a bit about yield.