Let's introduce a coordinate system with origin at the center of the bend, that is, the intersection of the green lines. (It is your failure to show that point, and radii from that center, that made it hard to answer your initial question; your 297.95 here corresponds to the presumed 46 there.)
I'll take x going to the left from there, and y going down, to that both will be positive; the axes will have the usual orientation if you hold your picture upside-down. I'll take z as going toward the viewer.
I'll describe the elbow in terms of the radius of the pipe itself, r = 318.5/2 = 159.25, and the radius of the arc along the center of the pipe, R = 297.95+159.25 = 457.2.
The elbow is part of a torus, which is most easily described in term of cylindrical coordinates; I'll define the distance from the origin in the xy-plane as \(\displaystyle \rho = \sqrt{x^2 + y^2}\). Then the equation of the torus is \(\displaystyle (\rho - R)^2 + z^2 = r^2\).
Replacing \(\displaystyle \rho\) with its definition, the equation in rectangular coordinates becomes \(\displaystyle \left(\sqrt{x^2 + y^2} - R\right)^2 + z^2 = r^2\).
Now, as I understand it, the point you are interested in has \(\displaystyle y = 263.7\) and \(\displaystyle z = 100\). Plug those into the equation and solve for \(\displaystyle x\); what I think you are asking for (your "x") is my \(\displaystyle x - R\).
The hard part is solving the radical equation. Expanding and simplifying a little, it becomes \(\displaystyle x^2 + y^2 + z^2 + R^2 - r^2 = 2R\sqrt{x^2 + y^2}\). To solve that, you need to square both sides, which results in a fourth degree equation. That will be solvable algebraically only with a lot of ugly work. The best thing to do is to solve it numerically.
I don't know what tools you have; I've been surprised that you need help at all, considering that you appear to have some sort of CAD software, but someone should be able to help you use whatever tools you have to solve the equation.