""Doesn't the small red wire only connect the large red wire to the box when it sees voltage?""

That is correct.

""And if you have 12VDC in the large wire, why would you have any less in the small wire?""

The large wire is wired directly to the battery post. The small wire has the CRANK and RUN wires which both have several connectors along their length. The RUN wire has these connectors plus it also feeds 2-3 other items. All of these connections equal voltage drops and the small red wire will never see 12v at CRANK due to all of these drops. All of these drops are why I am trying to figure out what the minimum voltage needs to be to make the MSD box happy during the CRANK mode. The RUN voltage will see apx 14 volts all the time, but the CRANK voltage depends on the voltage drops and the condition of the battery each time you start it up.

From what I have tested, I think that the minimum is around 10.5v to be safe. My car was showing 10.1v during CRANK and it would not start, three different times. Then I showed a 10.2v reading and it started.
Then I ran the CRANK ignition wire directly off of the ignition switch straight to the MSD small red wire and now I am reading 11.1v during CRANK and it starts immediately.

Note: The 10.1v/10.2v/11.1v readings were taken when the battery was about 85-90% charged.

Thank you for your response.

Note: Three days now and MSD has not responded.