Freon R22 is over $120 a pound. If you are 3 lbs low after the repairman adds labor cost you are going to have a bill over $500. Its time to let that unit go. Buy a new one. If you are out of Freon and need a new coil you are well over $1700. Buy a new 410a system. You are wasting money and killing the ozone. R22 dry units are JUNK . They aren't as good as the original and only have a 5 year warranty.