we often deal with these issues in stoichiometry. Using 98% purity 15 g of the product was used, then to have the same amount of sn in a sample of x grams sn 95% purity, you equate x to 101% and you will find 15.5 g of sn to use. theoretically the difference is very little. I would use it anyway.
but this is theoretically, I dont know ...