Question on Vision Screenings

Hello,
When we do our vision screenings- how many letters/symbols on a line can a student get wrong to fail? Or rather how many to get right in order to pass? :P I am very technical and I need to know the exact details of everything! This is my first year as a school nurse so please help! I tried looking it up in the Titmus manual but didnt find the answer. I am actually going to just use good ole Snellen for older kids and picture box for kindergarten.
Thanks!!!!!