I cannot comment about a saliva test, but having just a morning cortisol check is like driving into a gas station and not filling up with gas.
You need two checks in the same day. Ideally at 0800 and 1700.
And to know if your cortisol is really low, the twice a day check should be repeated say 2 weeks later.
For the record, on consecutive tests if you are at the extreme of the so called "normal" range ie very HIGH or very LOW then I consider that not-normal. Such was my case on 3 consecutive tests.