Ian,

I have created altitude
displays which use custom
pid constant value to
zero display to known
elevation.

A knob display linked to the pid
which could be spun on
dashboard to increase/
decrease the pid's value would
be most useful. Limiting by
unit 0% and unit 100%.

Altitude in Feet:

EWMAF(0.08:Lookup((1-((((Baro()+Val{Baro Adjust 0133})/0.145038)/Val{Barometric Pressure at Sea Level in kPaA})^0.190284))*145366.45::145366.45='---'))

Baro Adjust 0133:
internal pid used as constant.
used to set altitude to a known elevation.

Barometric Pressure at Sea Level in kPaA:
internal pid used as constant.
(101.325 kPaA)