Converting Geographic Coordinates into Lambert2008 coordinates
Since the code is about implementing mathematical formulas I would make an exception and completely ignore the usual naming conventions. Instead, I would use the exact symbols as  that document.

This would mean a little bit copy/paste because it's difficult to write symbols like [ϕ, λ, θ] but the compiler can work with unicode.

I find that code like this:

var rad = 0.01745329252;

var ellipsoide = (a: 6378137, f: 1 / 298.257222101);
var ϕ1 = 49.8333333 * rad;
var ϕ2 = 51.1666667 * rad;
var ϕ0 = 50.797815 * rad;
var λ0 = 4.359215833 * rad;
var x0 = 649328;
var y0 = 665262;


is the most easy one to understand because you can compare it with the book almost without any translation.