For example if a 10-day challenge ends in 2 days, am I 80% of the way, or do I need to step it up? Right now I do the math in my head, but is there a way to get the app to do that for me?
You have to do the math yourself.
Some self promotion: I use my web site which will show the progress and if you are on target or not:

It is called https://garminbadges.com/
If you upload your challenge data to the site you will get that challenge page with the current progress for you.