In the description of familiars, it says:
For the purpose of effects related to number of Hit Dice, use the master’s character level or the familiar’s normal HD total, whichever is higher.
The familiar has one-half the master’s total hit points (not including temporary hit points), rounded down, regardless of its actual Hit Dice.
So a wizard with 10 HD and 40 HP will have a familiar with 10 HD and 20 HP. Let's assume this familiar's base creature only has 1 HD.
What happens when the familiar wears an Amulet of Health +6? Should it gains additional health based on its original HD (so +3 maxHP) or the HD granted by being a familiar (so +30 maxHP instead)? Should this be considered as one of the "effects related to number of Hit Dice"?