TLDR: I pay for dates, not BCD. Once I embraced that mind set, the negotiation and expectation discussion is straightforward.
I make a point of discussing arrangements as full dates that can include multiple activities in addition to BCD. My allowance offer is for each date, which I expect to last several hours at a minimum. Dates can be longer if we do some activity that requires more time (ex: a special event, concert, festival, or a drive up the coast for wine tasting). If I want to plan an overnight event, I will offer additional allowance if asked, usually around 1x PPM per day.
Sometimes I take an SB to an event where the schedule or logistics won't easily accommodate BCD time. In those cases, I still provide her usual PPM, with no "make up" BCD expected later. But that's my decision to do. My approach: I'm not paying for sex like I will with a hooker. I am paying for her companionship on a date. And most dates include sex.
As others have mentioned, some SB's decline non-BCD time from the start, and some end up declining them later. That's their choice - remember that consent for anything can always be revoked at any time. But those arrangements that start or evolve to BCD only don't tend to last long for me, with that SB either ending the arrangement when she finds a bigger wallet, or with her being demoted to my B- or C-List of BCD backups.
Life is good
The Cat