It kind of depends on the circumstances, from what I understand. I'm still learning about this but have noticed a few trends. I judge whether or not a horse is jumping well based on its specific circumstances, especially for rare breeds:

Foundies with no DNA or items would probably get stuck in the 70's.
Fully itemed foundies seem to get up to the high 80's and low 90's.

For a purebred, un-itemed horse with decent DNA and stats, I think 90-95" would be a good jump.
For an average itemed horse (not a super), I would think 95-99" would be a good jump.
It seems like horses that get over 100" are usually supers with excellent DNA, excellent X-Ray score, and probably Lucky Charms.

Obviously there is more to it than I have just described, but hopefully that answers your question ;)

Thanks so much! That was very helpful. One more question. Is it better to wait until later to jump or start early? Or is it just personal preference?

Personal preference, in my opinion. I like to see my horses have more wins than losses in their jumpoff record, so I tend to wait until they are 8 or 9 to jump them, and even then not very heavily. I don't really start jumping them hard until they turn 11. We have learned from x-rays that horses peak from 11-18 years old, and beyond that won't improve anymore, so I try to reserve the high volume of jumping for that time :)

Some folks like to jump them early and identify which horses jump higher early on, which is totally fine, but in my opinion you're more likely to have more losses that way.

Logically speaking, you would want to wait until your horses have full loyalty, which I believe is somewhere in the 5-7 range, I forget exactly when.

I've jumped a few 2-3 year olds because I've entered them in Jumpoff shows for that age group, because I figure one jump won't ruin their record and it's an opportunity for the horse to win the Best in Show award.

Would be curious to hear how others do it, because I know everybody has their own jumping plan :)

Starting them once they reach level 11 or 12 in training seams to be better than right away. Although there is no right or wrong to this, play around with what works for you. Having fun at the game is more important than how your barn and even horses perform, If you are happy with the results you are getting, so be it. That is all that matters.