John writes:
If assigned IFR altitudes can be considered to have a tolerance of +/-
200 ft before you are "busted", what applies to published minimum
altitudes other than DH and MDA on approach plates? Most people will
say that you are not supposed to descend any amount below the DH/MDA
altitudes. Does this also apply to procedure turn altitudes,
transitions etc.? Is there a tolerance that is in effect +200 -0 that
applies?

In practical terms, whereas a decent of 100 ft below an assigned
altitude will cause no problems, is such a decent of 50-100' below a
published procedure turn altitude, transition altitude enough to get in
trouble?

[It's "descent", not "decent".]

The question for which I've never seen a definitive
answer is where is the reference point on the airplane
for measuring the altitude to which the MDA/DH applies?
It would seem that on the largest transport aircraft,
the cockpit would be at a substantially higher altitude
than the wheels on an approach. The wheels could all
but be on the ground while the cockpit is at 50 feet.
Visibility for the pilot is the controlling factor.
If a radar altimeter is the instrument determining
altitude for a low DH, is its reading compensated for
the aircraft attitude?