Responding to me own message here ...
> warn condition = ${if < {$message_size}{150K}}
> set acl_m9 = 1
> spam = nobody
> set acl_m9 = 0
> add_header = X-Spam-Score: $spam_score\n\
> X-Spam-Bar: $spam_bar
Changed to nobody:true to messages which are thought to be non-spam also get
acl_m9 set to 0
Also, removed entire add_header from here and added it to remaining warn
statements so non-SPAM does not get any X-Spam-* headers added.
Steve