General House Rules
You gain your full hit points at Level 1 and then 60% of the full HD for each subsequent level.
Example: When Eximere (Who has 13 Constitution) levels up from level 1 (9HP) to level 2, he gains 5 HP (8 * 0.6 + 1 = 5.8, rounded down 5)
You die when you reach 0 Con or are reduced below your negative constitution.
Example: Thundor (Who has Constitution score of 14) can die if his current Constitution is reduced to 0 or when his hit points reach -15.