01
Keep the units distinct.
W1 choke uses pixels. Edge margins use millimetres. They affect the white edge and canvas bounds respectively.
Set artwork dimensions and PPI first, then choose choke and margins for the order and proven production settings.
02
Inspect the actual file.
A 15 cm wide artwork with 5 mm added on each side gives a canvas about 16 cm wide. Review both sizes.
Open the output and compare W1 against the color artwork around edges, small text and fine lines before production.