The solution to this is probably really simple, I just can’t think of it right now and was wondering if someone could help.

I have a quad defined by four points in 3D space. It is planar, but the edges are not necessarily parallel. If I take them as two pairs, linear interpolate along both, and then between the results using a normalised X and Y value, I can get any position within the quad with those two values. Question is, how do I go the other way around? If I can get a point in 3D I know for sure is in that quad, how can I get the normalised X/Y? Whenever I try and solve it mentally and solution requires that I determine one axis using the result of the other, which of course is impossible.