ASTC - Fantastic! but...

ASTC compression is MAR-VEL-LOUS! But is there any reason why , when I use ASTC 4x4 block compression on my atlases, it distorts them? Also, it gives me the error "expectedImageSize + 16 != length in UnityEditor.DockArea.OnGUI when I’m developing for iOS. On Android, it gives me no error and all is fine in the editor, but once on the device, it is still distorted. Using 5x5 blocks is fine though.

Details: seems like it does not like when the image has sizes as a multiple of the “block size”

OK.

Now I was wondering, while doing my homework and researching about ASTC, how could I decide which block size use for my images? I mean, what should I consider to compress with 4x4 block compression or 5x5 block compression? Does it have something to do if the image is not square?