I've had a P85 for 4 years, and have always charged to 90%, leaving the car plugged in overnight and whenever I'm home. And have only used 100% a few times, and quickly started driving after the car hit 100%, so it wasn't at a full charge for very long. And, when driving the car has been below 10% only a few times - and only for brief periods at the end of a drive before reaching a charger.
At home, for most of the time I charged at 80A with my dual chargers - charging at home at the fastest possible rate.
And I've only lost around 5% of the original charge, which is consistent with what other owners have seen.
I've been at the 5% level for quite a while now. It seemed like the car lost that charge level and then stabilized, and haven't seen any further degradation (at least so far).
Losing 5% over a several year period is much better than I expected - so it's not clear, at least from my experience, that it's worth worrying too much about the charging practices, as long as you try to keep the charge level between 10-90% almost all of the time.
And, for software-limited 60's (which are really 75's), you may be able to charge up to 100% without any risks, because you still have the extra 15 Kwh of capacity that hasn't yet been unlocked.