That is exactly what I would have told you to do (I am also a rehabber). And calling her at 9:45 in this case was good, because owls are nocturnal, so that is when the mom would be taking care of the baby. Unless you saw an injury, there is no reason to think it wouldn't make itthrough the night - there are tons of baby owls right now doing justfine out there!
Just to give you an idea what it takes for a rehabilitator to raise a baby owl: Handfed mice pieces three times a day, to a bird that is scared of you; when he gets older, he has to be live prey trained so that the rehabber knows the owl knows how to hunt on its own. Sometimes they get it, sometimes they don't. They can only learn that from their parents, so if they don't get it, they can't be released, and then have to be placed somewhere. It is very sad, to never be free.
So, you did the exactly right thing - mom will take care of him & teach him how to grow up into a self-sufficient owl:biggrin2:
PS - This translates into all things wild! Leave the baby birds and mammals to their mom, unless you see an injury. Unless you see the actual mother on the nest be killed, don't assume that the animal you saw in the road up the block was the mother of what you found - it probably wasn't.