Rick
My 2 cents is to strongly echo Rotortorque' s last tip for ERPM
Mother Rucker used the OH-13T for Instrument transition (YES, simulated IFR, frosted bubbles and all in 1965)
The crusty old Warrant CFI's required us (especially brown bar lieutenants) to be able to call out engine RPM by sound alone..suction cup shield on my view of tach.
After a few days of calling, and comparing, it made the division of attention during maneuvers much easier, tach became crosscheck to fine tune, ears were primary
W-4 would carry a baton like wand, and whack my APH-5 helmet to interrupt my fixation and signal his desire for me to call out what I heard the engine RPM to be
Got so ingrained that the first couple oh rounds hitting my Huey in RVN (Post Graduate School for most 1965 Rotorheads), I was thinking my RPM was too high or low checking
Also, Why not a B2A....those egg bumps in the bubble over the pilot heads are real nostalgia items for us old gaseous guys