First off, measuring FOV in third person is really hard. Most TPS games don't state their FOV, so if I want to add them to the calculator I have to measure it. For this to be even feasible I need two things, the sensitivity to be accurate (i.e. no acceleration or smoothing) and I need a fixed point very far away (as a skybox or an object several miles/kms away). Both these things can be very hard to come by in some games. Some might have slight smoothing that makes measurements inaccurate, others might be TPS games in confined spaces that makes measuring the true FOV almost impossible.
Either way, converting between TPS and FPS boils down to two points, just like any aim. Do you want the same 360 distance, or do you want the same monitor distance. Both of them might feel different between the different views though, which is why it might be beneficial to have one conversion for first person games and a separate one for third person.
TPS games tend to have a narrower FOV because the camera is further away from the pivot point.