Continue to join the corset panels.
The center back panel should have the center back fold line marked in chalk or tailors basting stitches.
Once all panels are joined, pulling tension firmly take your first measure from busk waist level to the center back fold line.
The waist measure should be a maximum of 12mm (1/2″) longer than your finished waist.
When the seams are topstitched to the second lining the length will be reduced by this amount.
The final measure can be adjusted at the center back fold line.