On the PF sheet, you won't be able to calculate the hp of one character using the hp of another, AFAIK... You'll need to create a sheet for the familiar and give it half the hp of it's master. If needed, you can assign "None" for the HP's ability, you can also use the HP's Misc Bonus field. Also, if you make the familiar an NPC sheet, watch the Auto HP calculation on the settings page. Normally it's set to 50% if you check Is Monster or NPC. You'll probably want to turn that off. Manually track the familiar's hp as needed. Maybe create a macro to show what the current hp of the familiar should be based off of it's master ie " Familiar HP [[ floor(@{MASTERS_NAME|hp}/2) ]] "