[PF] HP based on Macro?

One of my players is going to play a witch and his familiar is suppose to have 1/2 the total HP of the witch. I was trying to figure out if there was a way I could do this by putting @{witch|total-hp}/2. I tried putting @{witch|total-hp} in the HP adj2 field or the HP field for class 0, but no luck. Any ideas? Manually adjusting it at level up isn't the end of the world, but would be nice to have it done automatically.

Those macro fields only look at other fields on the same sheet. We can't see other fields outside of the sheet.

Putting this in the HP adj2 field on the Core tab seems to add 1/2 level to total HP just fine: [[ floor( @{level} *.5 ) ]] EDIT : Never mind, I didn't quite pick up on the idea of using a different sheet's values.

Thanks for the response Chris. No biggie. It sounded like adding anew animal companion/familiar tab was on the road map.