Rays being rays, I guess that even with 10% of the water remaining they're still fully submerged ?
Prior to my aro I used to drain water until the fins of my fish touch both the substrate and the water surface. them being large angels meant that it was around 80-85% (according to my super accurate eyeballing)
angels being tall...doubt it be 80-85% unless you have small angels or very tall tanks
that being said, i am also changing 80-85% for my aro