This is an ECID/every caregiver is different thing. Overall, the objective is to get comfortable shooting lower numbers over time. This is where your data collection comes in, because that tells you how your own kitty usually responds to juice (keeping in mind, of course, that there can always be a "unique" cycle where a kitty doesn't respond as expected). Ideally, a kitty's cycles will "flatten" out, meaning that as you shoot lower, you will see nice flat curves rather than steep dives and climbs.
In general, the "rule" is to not shoot anything below 50 - there definitely is not enough wiggle room if a kitty does drop after shooting such a low number. I would never shoot anything under 50, and you do want to have a good deal of data and experience before you shoot anything in the 50s, or even 60s and 70s, and 80s.
Whenever you shoot your lowest PS number for the first time, you want to be extra-diligent about testing....you don't want to shoot and run out for the rest of the day. You want to have plenty of strips and HC food and Karo on hand when shooting your lowest PS for the first time.
There are other options, too, such as stalling (personally I was never a fan of stalling, as it was too challenging for us to get back on schedule), or shooting a one-time reduced dose, too, when you are facing a number that is lower than you've shot before.
Great job shooting today, and great job surfing it, Sami!!!