There is a rating system from 1 to 9 to describe the horse's
condition in terms of weight. Maybe someone else is also familiar
with or may I will recall where it is that I have seen it.
5 is about the mid range. 4 to 6 is acceptable. It is my
impression that show horses are kept nearer 6. 4 is reasonable
for an endurance horse.
If I remember right, 5 is about where the ribs have just about
disappeared visually but still can be felt reasonably easy.
At 6 or 7 a crease starts to form on the side of the spine. You
can also definitely feel fat back in the rump area.
As I indicated I am going from memory but perhaps someone else
is also familar with this system and can fill in the missing
details and perhaps calibrate more accurately the numbers I gave
and what is to be expected.