d20 Houserule XP Bonus Question Options

[Socinus]

I'm gearing up to try and GM a Pathfinder game for a couple of friends and I dusted off an old houserule that I'm particularly fond of but I've run into a problem.

4. Healer- I will not force a player to be a healer or cripple a party that does not have a healer. If the party and a player in that party agrees, they may take an NPC healer. That healer is the same level as the majority of the party and levels when the majority of the party levels, it does not get a cut of the XP. The backstory, characterization, and personality of the healer are up to the GM but the specific actions of the healer in combat are controlled by a volunteer player who plays both their character and the healer.

The healer may take defensive actions and heal damaged party members as well as limited other tasks ("Shut that door!" "Throw me a potion!") but no other tasks. It cannot be told to buff characters other than itself or otherwise cast damaging spells at enemies. The controlling player will recieve an XP bonus for their main character. The healer has a stat block, skills, and feats the same as any other character. The GM collaborates with the controlling player for leveling the healer. Control of the healer may be undertaken by one player for the course of the entire game or control may be passed around to different players. The healer may be equipped with items and be instructed to carry loads. The healer may be a mercinary and demand compensation or the players may be able to foster a friendship and have him accompany them for free.

This worked very well when I used it last but we also didn't use the XP system so I never worked out a good XP bonus for it.

What do you think would be a good XP bonus that would reward a player for extra work but not push them way ahead of other players?