I guess that any attempt to verify the decimal value of Lev Shestakov´s shared aereal kills in Spanish Civil War is hopeless task. There is even disagreement about his individual score. Some literary/web sources claim that his "full individual" claim score in Spanish Civil War were only 2 confirmed + 1 probable, others put it up to eight "confirmed" and some argue that it might have been as many as 11. So when even his "invidual score" is somewhat a puzzle it is even more difficult to find out how accurate the "shared score" actually was.
If you are really interested, you can try to browse Håkans Aviation Page about the airwar in Spain - see
http://surfcity.kund.dalnet.se/scw.htm - there might be some details on Shestakov. There are also some good books about Soviet pilots and air advisors in Spanish Civil War - these surely have something on Shestakov and probably tell also something about his "kill score" in Spain.
Anyway I would not compare Shestakov´s score with those of real top aces of Spanish Civil War. For example J. García Morato´s 40 air victory record was based on indivdidual claims and is therefore more clear stuff than Shestakov´s 2½/8/11 individual (?) + 31 shared (?).
There is also the further issue of comparing the claims with recorded enemy losses. In that case J. García Morato´s score would be easier and more thankful individual case study than trying to find any "true" decimal value to Shestakov´s shared kills.