Was just helping a client whose site audit revealed that their meta titles were too long, and found this great summary resource. Their site also had 200 pages where no meta descriptions were present. A big site, lots of blog work over the years, and I suggested a way their developer could bring in the first 160 characters of the blog post or page’s content so that at least the page had something.
It’s always better to hand-choose the words for a meta description as then you have full control.
In terms of on-page SEO priorities, it’s most important to have an SEO phrase ranking goal for that page or post or product. Write it down somewhere in Google sheets so you can sort later to make sure you don’t do the same phrase for 2 pages.
Then, get your ducks in a row: Ensure the phrase is used in the Heading 1, Ensure the phrase is used in the page copy 2-3 times. Ensure the phrase is used in the meta description. Ensure the phrase is used in the image filenames and image alt tags.
Have a read of this: https://moz.com/learn/seo/on-site
Contact us if you need more info and help to get your site ranking.