Looking at the NHTSA there seems to be conflicting info. One spot it states that the system should indicate when any tire including all four drops 25%. In another spot on the site it seems to be referencing a table that states 20 psi is the safety floor. Although it does state
"Overall, we feel that the 25 percent threshold adequately captures the circumstances at which low tire pressure becomes a safety issue. We also believe that this level would be acceptable to most drivers and would not be considered a nuisance to the point that it would be ignored by large numbers of drivers. We also believe there is no reason to examine higher thresholds (e.g.,� a 30 percent threshold), which would provide fewer benefits for similar costs"