A 45-year-old needs an indirect composite onlay on 36 after removal of a large MOD restoration. The distal box margin lies 1.5 mm below the gingival margin, but a sectional matrix seals well against it, and bone sounding shows 3.5 mm from the margin to the alveolar crest. The tooth is vital and asymptomatic. How should the subgingival distal margin be managed before the impression?