I believe that with NO vacuum at the vacuum modulator, the trans pressure will be maximum. It's at this low/no vacuum that the internal pressure can be set too high. As I understand, the pressure is set to a given value, which will produce firm, very firm or extremely firm shifts depending on clutch condition, etc, and the vacuum is used to 'soften' the shifts.

Have a look at this article, considered to be The Source for transmission adjustments.

http://www.continentalimports.com/ser_ic20242.html

Best Regards,
Jim