You can inherit the write method of that object, something like this:

class class_name(osv.osv):
    _inherit='object.name'
    def write(self, cr, uid, ids, vals, context=None):
        #Maybe some code here
        if vals.get('state'):
            if vals.get('state')=='done':
                #write your code here to write the percentage

    return super(class_name, self).write(cr, uid, ids, vals, context=context)