What ratio seems to yield the best match to the true magpul FDE color using the Cerakote Magpul FDE? Did a couple test pieces tonight and they came out darker.

Ratio of hardener will only effect sheen, not shade of color.

It is usually pretty spot on for me. You could have cooked it a little hot or too close to an element because that is a pretty big difference.

Then I compared it to a factory FDE spikes lower from their lower/upper set and the colors are nearly a perfect match.

Could be in the stock then.

Also keep in mind, different methods of coloring will often trick the eye. Light can change shade just by the substrate.
For example, a dyed fabric will look different than a painted smooth surface. The way light is reflected off the object could change the appearance slightly. So it could just be a trick with the light.

Might try taking it outside or under some different lights to see.

If all else fails, you could paint the stock too.

Magpul FDE is my favorite color and there has only been a couple of times that Cerakote (both H and C series) has matched Magpul's furniture.