First off automated domain appraisals are usually wrong and by a lot. People who have been in the domain business and are experienced domain investors (domainers) will not use these and look at their valuations as a joke. I have been a domain investor since the mid 90's and have seen many people buy or try to sell domains based on the automated appraisal sites like estibot, only to find out that the estimate was WAY off.
Automated appraisals are not accurate. There is no rhyme nor reason for their appraisalsand if you don't believe me that they are way off go to estibot and check nfl.com $0, mlb.com 1.8 million dollars, there's a good laugh. Godaddy.com, $0.
Second, there is no accurate way to calculate a domains value exactly. There are so many factors that contribute to the value of the domain. There is the extension of the domain (usually most important), how many characters the domain has, does it have numbers in the domain, does it have hyphens, is it easily pronounceable, does it mean something, does it have a popular word in the domain and many other factors.
The best way to find out the value of a domain is to get it appraised by people who have experience in this field, a lot of experience. Many of the domain selling platforms offer domain appraisals but they usually give an appraisal much higher than the actual value in hopes that you will list it in their marketplace for sale.
Best way to find out what a domain is worth is to check with experienced domainers!