Some estimates...

Assuming that the convex surface of warping is spheroidal and assuming that Chris' measurements for warping on his 40x10x20 mm test block were taken at the extreme midline rather than the corners I come up with a radius for the spheroid of 425.3 mm for the warping of 0.47 mm for his 50% fill HDPE test block.

If that is meaningful then if Chris printed a 40x40x20 block of HDPE at 50% fill one would expect that the curling at the corners which would have a distance from the block's centroid of 28.8 mm would be about 0.94 mm.  The warping at the midline would remain at 0.47 mm, of course.