The AP live data monitor list I look at has 'boost' and then 'boost target' right under it, TD boost is further down the list. I'll take your word that -8 (16 inHG) is the correct reading at idle but I previously assumed that the AP's boost reading should match its boost target reading at specific rpms/throttle.
The boost has fluctuated with stage 2, it just didn't happen when I took the above datalog. I'm not sure yet what conditions induce the fluctuation; I'm pretty sure it does it in hot and cold weather.
I'll have some time to mess with this tomorrow, will do as you advised then report back