It was a difficult question for me to answer a few years ago when I began this FF adventure. At some point you just have to place a stick in the sand and adjust your perception from there depending on how things go and what you do travel wise.
Despite my best efforts and wishes, most of my travel ends up being domestic and I'm pretty fond of VA flexi->J points upgrades as a result ... this reaps me a perceived value around 2.5-3c/p usually. I'm unlikely to part with points for much less than 2.5c each.
I understand the concept sometimes floated of only comparing those fares you'd actually buy with cash but for me its a little more complicated than that. For example, for leisure travel I'm fairly unlikely to stump up the $3-4K return pricing required for PER to the east coast in cash ... yet, in my perception there is a wide chasm between Y and J travel classes, even on the 332 where Y isn't all that horrid.
I'm probably the classic champagne taste / beer budget leisure traveller (hehe), so happy to pay the somewhat overpriced Flexi ticket price + 10K points to upgrade to J ... but how does one fairly and reasonably look at the value of those 10K points? Redeeming against domestic Y fares typically returns <1c/p in value, so if thats the measure, then the J ticket at say 500 for flexi + $100 in points value = $600 one way PER to say SYD in J ... but there are no tickets in J available for that price, whether a J ticket is _really_ worth $1500 (or whatever) is purely conjectural because the cash asking price _is_ $1500.
Originally I went through a large exercise of building routes and comparing pricing for a host of likely to take trips, then compared the J asking price in $$ to the cost in $+ points or even just points. After a lot of work it started to appear that the redemption value was falling around 3c/p for me ... sometimes higher, occasionally lower, but it was a reasonable starting point. I've not really had to readjust that figure for a couple of years now.