The length of a continuous placket is calculated by measuring the length of the opening on the garment where the placket will be inserted.
Typically, this length includes:
The measurement of the garment opening from the neckline or collar down to the desired placket end point (usually around mid-chest or wrist, depending on garment type).
Additional allowance for seam and finishing edges, usually about 1 to 1.5 cm on each side.
Hence,
\[
\text{Placket Length} = \text{Garment Opening Length} + \text{Seam Allowances}
\]
This ensures the placket fits well and allows easy garment wearing/removal.