There's quite a bit of misinterpretation going on in this thread.
To start with, she doesn't convert the mass of the black hole into energy. She states, quote, " felt electromagnetic and gravitational fields wanting me to push them around, felt fissile reactions building, waiting to move through me". They then say that she is "converting energy from proximate singularity". Captain Marvel is not converting the black hole into energy, she is absorbing energy and converting it into energy blasts.
In addition, what they say is not "she has converted 81% of the black hole", what they say is "81% efficiency", with the previous lines clarifying the context behind this as being the efficiency at which she is "converting energy from proximate singularity".
Essentially, what this means is that she is absorbing and converting energy from the black hole with 81% efficiency. If the black hole is providing her "1,000,000 Joules" to absorb and convert, then she would be blasting the Builders with "810,000 Joules" in turn.