You’re simply wrong about this.
I studied up to a masters in physics, I've seen the math, I don't believe there is a way to calculate that, there is no way to compare time inside with time outside.
Edit: I'd love to see such a formula if there is one btw, so if you know there is one please link!
When you say "thing", it's more like photons. While thought experiments of a diver falling into a black hole are fun to ponder, nature isn't so gentle.
The corona, tidal forces, magnetic fields--and other factors--would obliterate matter into a stream of subatomic particles and light before crossing the event horizon. So the concept of a "thing" moving inside a black hole is likely misguided.
It’s been called one of the most extreme physical environments in the universe.
Strong magnetic fields threading the inner accretion disk extend out of it,
creating a tenuous, turbulent, billion-degree cloud. Particles in the corona
orbit the black hole at velocities approaching the speed of light. It’s a source
of X-rays with much higher energies than those emanating from the accretion
disk, but astronomers are still trying to figure out its extent, shape, and
other characteristics. [0]
A few resources to check out: Sean Carrol, PBS Space Time, Ask a Spaceman, and Why this Universe are all great resources--and frequently do deep dives into this topic.