Wow. Interesting question.
To be able to do this you would need spectral measurements, not Lab measurements. You would also have to know something about the opacity of the ink used, and also factor in the optical brighteners in the paper. Then, since dot gain would not be linear, you’d have to take into account that when you ask for 50%, you will actually be getting something else (say 60%).
So, no - we don’t know of a way to do this in a mathematical formula sort of way.