SKSE 2.2.6 works perfectly with game version 1.6.640.
Are you planning to your game from the latest version to 1.6.640, or are you setting up a new mod list from scratch? skse 16640 link
Look for the "Current Anniversary Edition build 2.2.3 (game version 1.6.640)" link. SKSE 2
These point to the which is the exact match for game version 1.6.640 . skse 16640 link