Originally Posted by GUWonder

I most frequently don't have PASSID in the KTN fields at the time of bookings and yet I and my other travel companions with PASSID numbers reliably get PreCheck LLL outcomes by having that number entered in at some point before final check-in and clearing security.

I've used the KTN field with PASSID even 40-180 seconds before check-in, and PreCheck LLL has shown up very reliably in such circumstances for us -- and more reliably than when the KTN field was filled in days or even weeks before. That, however, is when checking in with and flying US carriers (domestically in the US or from the US). I've even had it work highly reliably after checking in by getting the passengers un-checked-in and then getting rechecked in after the KTN field is filled in with say the PASSID.

The KTN field need not be filled in at time of booking, but it should be filled in before you get the boarding pass you intend to use to clear security when flying from a PreCheck airport with a PreCheck-enabled carrier.