Peter Cole said:
I would like to know exactly how black holes convert some of their mass into energy.
The article you gave a link to in post #4 gives the following paper by Martin Rees as a reference for the 6% and 42% figures:
https://www.annualreviews.org/doi/abs/10.1146/annurev.aa.22.090184.002351
As the article says, and as the paper discusses in more detail, for a non-rotating hole, the 6% figure comes from gravitational binding energy, considered as a fraction of rest mass: basically, we imagine an idealized process where an object with some rest mass ##m## goes from rest at infinity into a circular orbit about the black hole at some radius ##r##. This process requires extracting energy from the object, and if we pick the radius of the circular orbit appropriately, we can maximize the amount of energy that can be extracted. That amount is 6% of the object's rest mass, corresponding to a radius of 3 times the Schwarzschild radius of the hole. (Once the object is in that circular orbit, in the idealized process, no further energy can be extracted from it; it will just stay in that orbit until some small perturbation kicks it towards the hole and it falls in, but that process yields no energy.)
The 42% figure for a rotating hole comes from applying a similar argument but using parameters for Kerr spacetime instead of Schwarzschild spacetime, in the limit where the hole is rotating at the maximum angular velocity it can rotate and still have an event horizon. The closest stable circular orbit approaches the horizon radius in this limit, which is why the percentage is significantly larger than for the Schwarzschild case.
(Note, btw, that Rees' notation might be somewhat confusing. He uses the notation ##r_g## to refer to the quantity ##GM / c^2##, the black hole's mass converted to a length. It is more typical to see a notation like ##r_g## or ##r_s## used to refer to the Schwarzschild radius of the hole, which is ##2 GM / c^2##, i.e., twice as large as Rees's ##r_g##. In the above I have phrased things in terms of the Schwarzschild radius.)