There really isn't any more to it than that.
The fitness of the prescription depends entirely on whether you tell the optometrist or assistant that you can see the chart better with "side one" or "side two." Experience of the optometrist has little to nothing to do with it since the prescription is based on your judgment of which lens combination gives you better vision. Most people can be corrected to 20/20 and they will tell you what your reading is if you ask at the time. It should also be recorded in your chart.
Another trick is to schedule the eye exam at the time of the day when you typically have vision problems. If you routinely see fine in the morning but start to get waning vision by 3 in the afternoon, then the late afternoon is when you want to schedule the examination.
What you should always do though is take a new pair of prescription lenses back to the optometrist to make sure the prescription is filled properly. Sometimes it isn't, and in that case they will need to remake the lenses at no charge to you.
If you haven't had an examination recently by a medical eye doctor (an ophthalmologist) that might be an option. There could be something going on in addition to just refraction that needs checking. I'm thinking of things like dry eyes that can affect the quality of vision. So can a lot of medications and some medical conditions like diabetes. For example, if someone's diabetes isn't well controlled, and they notice poorer vision later in the day, it may just be that their blood sugar was high. There are many, many things that can affect vision, so having a medical doctor check you out at least every few years is a good investment.
Please let me know if you ahve questions,
Schuyler (blind as a bat since age 5 without corrective lenses)