OK sounds to me the issue is the voltage supply to the regulator, which is the same line as your ignition
The path for the "dark blue" ignition run is thus for a factory harness:
Battery -- starter relay -- fuse link -- bulkhead connector -- ammeter -- in harness splice -- ignition switch connector -- through the switch -- back out the switch connector on the dark blue -- back out the bulkhead on the dark blue -- and then to
alternator field (blue)
ignition system
idle solenoid if used
electric choke if used
smog doo dads on some cars
and most important, the regulator IGN terminal. You show you have nearly 1 whole volt drop (.8v) and this is very likely the culprit.
You need to go through, and your most likely suspects in probable order are
fuse link line (red) bulkhead connector terminal
dark blue bulkhead terminal coming back out
ignition switch connector
the switch itself
more rare but sometimes the ammeter connections or.........
the ammeter itself
more rare but does happen is a loose "in harness splice"
Here:
is a good article, and the simplified diagram shows most of this, except for ignition switch and ignition run buss (dark blue)
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
The ignition switch is fed off the "welded splice" which is a factory splice, taped up in the under --dash harness in the black ammeter wire a few inches away from the ammeter
One way to fix this is to cut the dark blue after it comes into the engine bay and use it ONLY to trigger a relay, then use the relay to feed the regulator/ alternator field/ ignition under the hood