For a 90% CI there is a 10% chance that the answer lies outside your estimate, and if you split this there is a 5% chance that the answer is above your upper bound and a 5% chance that the answer is below your lower bound.

This isn't always true. For example, one calibration question I've done is, "How long are all 3 extended Lord of the Rings movies back to back?

On this, I was almost certain they were at least 3 hours long, but I wasn't sure how much more than that they were. So, my minimum was 9 hours. I was fairly confident they weren't more than 4 hours, so my upper was 12 (this was for a 70% interval). Almost all my uncertainty was on the upper end, while very little was on the lower.