Speaking of alternate approaches, there's an interesting and fairly simple technique for measuring both R and the absolute value of X. I don't think it's anything new but I don't see it discussed much. It can be done with simple diode detectors, but I wonder if log power chips (AD8307) would help or hinder?
I built a version several years ago helping a guy on a commercial project. It could be pretty good if the values didn't get too extreme. But I don't remember exactly what "extreme" was.
A working product with some explanations is VK5JST's Aerial Analyzer:
A paper on the math an a simple example circuit is here:
I can't remember if the MFJ259B uses this technique or not. Probably does.