To tell you the truth I have never paid much attention to actual readings. I only pay attention to relative readings.
I am sure that idle in park should give the highest reading. Full throttle would give minimum readings. Any condition that reduces engine speed at constant throttle will reduce the reading. Thus at idle as you add load (trans or A/C) the gauge reduces.
I have a fair idea what the actual vacuum readings would look like at various load conditions but the economy gauges aren't numbered gauges and probably vary from car to car a fair amount.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician