^ @WayOutWestMom - perusing the UCIHP site fairly quickly, it appears that UChicago college students interested in applying to a health profession grad school (including medical) are invited to participate in a two-year application cycle, the successful completion of which will culminate in the HMC rec. letter. Those a few years out of college may not find the rec. letter all that helpful given their other post grad experience.
https://careeradvancement.uchicago.edu/uchicago-careers-in/health-professions/applying
Best guess is that post-bacc can be discussed with the preprofessional advisor who has an idea of what sort of GPA would be advisable for that student. UChicago college matriculants or current enrollees who are interested in pursuing a health profession are assigned their preprofessional advisor as soon as they indicate interest. For many that means before they even show up on campus as first years.