SO2 testing and retesting

[Zog]

I finally broke down and purchased a Vinmetrica 300 in hopes of getting a better handle on SO2 and pH. So, I tested a sample of cider wine today and came up with 8 ppm. Using the sulfite calculator at winemakermag.com I plugged in my numbers and it told me to add 1.2 grams to get it up to around 40 ppm. I stirred in the sulfite and shortly after I drew another sample and retested. It was only at 20 ppm. Should I wait longer before retesting? Thanks.