Add Influence
 

Hi,
Don't know if this is a newb question. It has to do with skinning. If there is a more appropriate section to post this, please advise:

I have a completed, weighted character mesh, now I want to add a little bone under the arm to move the back very little.

When I use the "Add Influence" option - I've tried every setting.
It puts a lot of extreme weighting on this little bone and ruins my weighting in the arms, shoulders, back, etc.

Does anyone know how to add this new joint into the influence of the mesh with just the bare minimum of weight, so I can slowly, gently Paint Weights onto it and carefully grow its influence in a small area?

Please help, I'm running out of time. Thanks in advance.