3 yr old Nutone/Nordyne 3 ton Heat pump, system has a slow leak, after two years needs recharge, just got bottle of 410A, have means to weigh bottle as I go. Can I just add liquid 410A into low pressure fitting and go by feel and gauge set. Or do I really need to read high side temp differential and add vaporized 410A? Don't have ready access to a Multi meter with temp or a vaporizer, guess I could source them. Thank you.