before you start any extraction, you should evap-test all of your solvents for impurities.
place a few drops of naptha on a clean glass dish and leave to evaporate. if there is any residue remaining, you should not use that brand of solvent.
i assume you're talking about after the freeze precipitation step, and i'll assume you know what to do with the mother liquor...
when you extract and filter the DMT crystals from the mother liquor, you should place the wet crystals onto a clean glass dish to dry all the remaining naptha. it is best to cover your dish with a dry lint-free cloth to prevent dust and/or bugs from getting stuck to your product. after they appear dry, gently crush the crystals into a fine powder. stir the powder and continue evaporating until all naptha smell is gone. then you can weigh your product and determine your extraction yield.