1.022 to 1.026 is normal for FO and FOWLR tanks albeit there is no good reason to have it any lower than 1.025, where reefs should be 1.025 to 1.026.
Best bet it to either buy a refractometer $50-60 and periodically recalibrate your hydrometer (I've seen them off by as much as 0.008), or take a water sample to your LFS and get them to test with a refractometer so you can figure out how much yours is off by. I have two hydrometers (one is SeaTest and one is Coralife...I think); one is under by 0.002 and the other is over by 0.002.
For a really rough guestimate...about 2 tbsps will raise the sg by about 0.006 or 7 ish in 5 gallons. This is just my guestimate from mixing salt up in my buckets.