I’m sure we are on the same page here but:

for clarity. You may begin an FDP that could potentially show more than 138 hours of duty since your last 30 hour break as long as that FDP *began* prior to the 138 hour limit that would necessitate a new 30 hour break (138+30=168, 30 of that being the rest) prior to a new FDP

Assume you’ve had 30 hours off and you begin at 1200E today (9/11). You can operate FDPs that begin as late as 0559E/17th (+137 hours 59 minutes) within an intervening 30 hour break. That final FDP beginning at 0559E may intrude into where you might expect the 30 hours to begin. However I once that FDP ends you now need a prospective 30 hours prior to any new FDPs.