I don't think your current approach will get you what you want. Using an average on all evolved Pokemon will get you...well, an average evolved Pokemon. Evolution can entirely change the way a monster is used. I would recommend using the information from Pokemon builds that fall within a single class and possibly further narrow your numbers to a particular tier. Also, you may want to keep in mind that stats aren't the only thing that determine the usability of a monster. In case you don't know about classes and/or tiers, I'll include some more info below:
Pokemon "builds" can be grouped according to their function in battle (sweepers, walls, annoyers, healers, etc.). The following all work together to determine the class of a given monster:
- move pool
*These can change after mega evolution or because of other items like: Eviolite, Light Ball, Choice items, etc. (Stats can also change based on breeding and nurturing, but the details are too much for this answer.)
Competitive battling communities (such as Smogon) have grouped Pokemon into Tiers depending on their usage for each generation. These are commonly seen as Uber, BL, OU, UU and NU. You could possibly use these groupings alongside the class to better focus your numbers/builds.