Celestine, the Living Saint

Flying, lifelink
Healing Tears — At the beginning of your end step, return target creature card with mana value X or less from your graveyard to the battlefield, where X is the amount of life you gained this turn.

Warhammer 40,000 Commander

Celestine's triggered ability looks at the total amount of life you gained to determine X, regardless of whether or not you also lost any life. For example, if you gained 3 life this turn, X is 3, even if you also lost some amount of life in the same turn.

The target creature card is chosen as the triggered ability is put on the stack. Gaining life after that point won't let you choose a target with a higher mana value.