Lumps
I think others have covered that if you are in feather you have zero thrust, not simulated zero thrust.
From my experience I think 12" is probably a fair bit less than zero thrust. I have operated a 55 extensively and I have feathered one several times and they had reasonable performance.
At low RPM, manifold pressure is not your primary indication of power, RPM is!
(Remember below about 1800 RPM the CSU is not governing so basically your prop is behaving like a fixed pitch prop and on the B55 it is in course pitch at lower RPMs as well!!! No matter where your propeller control lever is except for the feather detent.)
For training we used about 1,400 RPM and propeller control in full fine pitch... we came up with this after firstly checking with the manufacturer and then verifying performance with one actually feathered. I think 1400 RPM was slightly worse than zero thrust but served us well for training.
We also used about 1300 - 1400 RPM on Barons, 310s, Chieftains and 402s all verified after testing.
I came across this excellent Cessna explanation from the C303 POH, but it is also relevant to most IO and TSIO 520s, and I'll bet it is pretty close to the IO-470s in your B55
"Propeller Control - Full Forward
Throttle - Adjust for RPM as below;
2,500ft & 10C at VSSE (80KIAS)- 1,150 RPM and at VYSE (97KIAS) - 1,450 RPM
5,000ft & 5C at VSSE (80KIAS)- 1,250 RPM and at VYSE (97KIAS) - 1,550 RPM
7,500ft & 0C at VSSE (80KIAS)- 1,350 RPM and at VYSE (97KIAS) - 1,700 RPM
10,000ft & -5C at VSSE (80KIAS)- 1,450 RPM and atVYSE (97KIAS) - 1,800 RPM
Add 50 RPM for each 15C above standard and subtract 50 RPM for each 15C below standard"