This is the base value for each level keystone when completed exactly in time. If you complete it faster the value will increase from here, and if you go over time the value will decrease.
Base Score will then convert into a Mythic+ Rating depending on whether it is your best run overall or your best from the opposite affix (Tyrannical/Fortified).
When a key is your Best Run for a dungeon it is worth 150% of the base score. When it is your Best Run for the opposite affix for that dungeon it is worth 50% of the base score.
|Keystone Level||Base Score|