Wither Bar | Edit health display with command

Hello,
I want to spawn / edit a Wither via the /entitydata command.
The health displays a full life Wither but I want to make him a half of that.
For example:
http://puu.sh/tR32w/772ff60061.jpg
This is a full life Wither.
But with that Command
/entitydata @e[type=Wither,r=10]

{Health:15}

the display should be not full.
I dont have any idea to fix this with a command.

Short: I can not edit the life display of the Wither with an command.

Sorry for my bad English.
Greetings from Germany