Example of Garen's Ult potential vs actual damage
Renekton has 4332 total HP. When Renekton is at 1332 HP, Renekton dies to Demacian Justice. Death recap and damage done is shown at 1725 true damage.
Since Renekton is missing 3000 HP, Garen's ult damage is 525+0.4*3000=1725. However, only 1332 damage was needed to kill Renekton. So, the potential damage vs actual damage is 1725 vs 1332.