What is the percent flattening in Haumea?
That would be because Haumea is a Triaxial ellipsoid. It's dimensions are different in every cardinal direction. The term 'flattening' only makes sense for the biaxial ellipsoids, because two of the axes are the same and one is different (in the case of Pluto, the equatorial axis is 1% larger, hence 1% flattened). The concept of 'flattened' doesn't make terribly much sense when all three axes are different.

Haumea's axis ratios are approximately 1 : 0.75 : 0.5, if that's the information you're after.