Where to set the bar?

It had been a nice day. Pancakes with fresh strawberry sauce and bacon with the girls. Yummy coffee and then off to church. It was on our way home that we hit the snag. Some how, even having had coffee, I was exhausted so we decided to take the slightly longer scenic route home through the amazing redwood forest. There is something about looking at the light in the trees that is calming and energizing to me and the girls seem to enjoy it too. They were, until Claire lost it and started screaming and biting herself out of nowhere. My blood pressure shot up pretty fast but somehow I managed to stay calm for the 10-15 minutes of intense screaming. I suspected that it was because she had to go to the bathroom. I told Claire that if she would calm down, we would take her the second that we got home, but that if she continued to scream, she would get a time out once home before going to the bathroom. Suffice to say, we got home, she had her time out, I hid in our room with my good friends Ben and Jerry and eventually Captain Awesome talked with Claire about the time out and took her to the bathroom. As it turns out, she was dry, and she voided while sitting on the toilet, which confirms my two suspicions. 1. she was upset because she had to go bad 2. Claire is VERY  capable. This is something that I really struggle with. I often wonder how often I set the bar too low or expect too little of her. Most often it is selfish. Like when she is screaming in the car and I know she will calm immediately if I put on her favorite music, but I am pretty sure that would only teach her that I am a puppet that is easily manipulated. Then there are times that I am afraid to try to have a high expectation for fear that I am just setting her up to fail because she has a very legitimate disability that doesn't let her always do what she wants to. For example, I wouldn't want to say, hold it until we get home, because she really might not be able to. I am curious, how have you dealt with this? Do you ever find yourself feeling like she is capable of so much if you could just be better at supporting her?