The rule
Read out of the game's own script (r4Game.ws, OnGiveReward) in the Remastered build:
| Geralt up to 9 levels above the quest | 100% |
| Geralt 10+ levels above | 5% |
| Geralt below the quest | 100% |
| Hearts of Stone: each level under | +16% (cap 150%) |
| Hearts of Stone: each level over | −16% (floor 5%) |
| Blood and Broken Bones / Death March | ×0.9 / ×0.8 |
| Quest with no level | always 100% |
Levels cost 1000 XP each. Kill XP falls off by level difference too, but never matters as much as the quest reward.
Why this order. It is ChrisRi's nothing-missed order. Checked against the rule above it forfeits about 28k of 86k quest XP; an order with no story constraints at all still forfeits 24k, because levels 22–30 hold more quest XP than there are levels to spend it in, and Blood and Wine's main quests out-level its side quests whatever you do. Every big move tried (Kaer Morhen earlier, Hearts of Stone earlier) cost more than it saved. Rows marked structural are that squeeze, not a mistake.
From a save
Drop a .sav here or . Saves live in Documents\The Witcher 3\gamesaves. Nothing leaves this page.