How do you calculate enclosure size with passive radiator?

I feel like I've looked everywhere but can't find an answer to this. I think I saw somewhere that the volume of the enclosure would be the same as that of a sealed enclosure but adding a little extra to make up for the volume taken up by the PR. is that right?

I'm asking this question as a followup to an earlier post. I just posted another question there but I may as well ask it here too. I'm converting an old radio to a bluetooth speaker so I have to build an enclosure that will fit inside of a 13.5x7.5x8" space. Based on that size do you think I could go three way mono with a port or PR on the sub, or will it be better to go two way mono with a port or PR on the full range?
Just run numbers on a standard QB3 or BB4 alignment.
Then with passive your trying to weight the radiator to achieve same tuning.
With passive the cut off rate is steeper so sometimes a fine tuning is needed.

Far as gross and net volume. yes
same as any reflex. the driver and anything else in the box has displacement which removes volume.
So you make the box slightly larger to make up for lost volume from, drivers, braces, ports, passive.

you calculate within reason the volume of the items inside, to determine how much larger the box needs to be
The passive radiator replaces the port in a ported system, but instead of adjusting the port length, you add or remove mass from the passive radiator to tune it to the correct frequency.

But I expect there exists a full-range driver that will behave nicely in a sealed box of that volume. That's far simpler, and may sound better since a sealed box has a slower roll-off and better transient response.