S = πAkc³/2hG

Where,
S = Entropy of a Black Hole
A = Area of the Event Horizon
k = Boltzman’s Constant
c = Speed of Light
h = Planck’s Constant
G = Newton’s Constant
G = 6.674 08 x 10⁻¹¹ m³·kg⁻¹·s⁻²
h = 6.626 075 5 x 10⁻³⁴ J·s
h = 4.135 669 2 x 10⁻¹⁵ eV·s
k = 1.380 658 x 10⁻²³ J/K
k = 8.617 385 x 10⁻⁵ eV/K
c³ = 2.69440024 × 10²⁵ m³/s³

Entropy of a Black Hole
https://physics.stackexchange.com/questions/387304/magnitude-of-black-hole-entropy

To calculate A the Area of the Event Horizon, A can be calculated….
Black Hole Parameters
https://space.geometrian.com/calcs/black-hole-params.php

Classical and Quantum Black Hole Cross Section
https://www.researchgate.net/publication/222692263_Classical_and_quantum_cross-section_for_black_hole_production_in_particle_collisions

Total Destructive Interference Nodal Points and Dark Matter
https://www.quora.com/profile/Rickey-Estes/Electromagnetic-Field/Dark-Matter-and-Dark-Energy-1?share=feecb3bc&srid=5BcRO

Short Wavelength High Energy Length Contracted Electron Cross Section(s)
https://www.quora.com/Is-length-contraction-only-a-result-related-to-observation-or-a-real-contraction/answer/Kirsten-Hacker?share=25fe22f5&srid=5BcRO

https://www.quora.com/Is-length-contraction-only-a-result-related-to-observation-or-a-real-contraction/answer/TR-Livesey?share=595ed886&srid=5BcRO

Phillip Freeman has a good answer to the question…
What is an annihilation cross section?
https://www.quora.com/What-is-an-annihilation-cross-section/answer/Philip-Freeman-3?share=e2b1a7ae&srid=5BcRO

The case for mini black holes
12 November 2004
https://cerncourier.com/the-case-for-mini-black-holes/
http://superstringtheory.com/blackh/blackh5.html

…and finally, I decided to ask quora users for more information, due to numerous conflicting results. I hope that this helps you understand and be able to accurately calculate the entropy of a black hole, for a referrence and as a study tool…
“Can you recommend a good source of information for calculating the entropy of a black hole, with examples worked out step by step?”
https://www.quora.com/Can-you-recommend-a-good-source-of-information-for-calculating-the-entropy-of-a-black-hole-with-examples-worked-out-step-by-step?share=9d716592&srid=5BcRO