Constant screeps::constants::extra::CONTROLLER_DOWNGRADE_PROGRESS_RATIO

pub const CONTROLLER_DOWNGRADE_PROGRESS_RATIO: f32 = 0.9;

Percentage of progress toward next level controllers retain when downgraded

After a downgrade, the controller loses its previous progress toward the next level, and has its progress set to 90% of the amount needed to upgrade back to the level that it just downgraded from.

Code reference