Based on my understanding of the image selection heuristics, a value of 63 would work like this:
image+63.png (exact) no match
image+65.png (increment of 5) no match
image+60.png (increment of 10--60 is closer than 70) no match
image+60.png (increment of 20--60 is closer than 80) no match <-- I'm not positive of this one; it may skip 60 and go straight to 80 since it's already tried 60.
image+75 (increment of 25--75 is closer than 50) match.
I agree that increments of 5 would be the best approach and yield the closest match most of the time. I have some images with 360(!) options to always get an exact match on the value. For example, my wind rose image has 360 images.