The three numbers
Finished face W × H, return depth R, and material thickness t. Everything else is arithmetic: blank = (W + 2R) × (H + 2R), with an R × R square notched out of each corner. The fold lines are one closed rectangle inset R — which is exactly the finished face.
A 600 × 450 pan with a 25 mm return is cut from a 650 × 500 blank, and needs 2100 mm of groove. Nothing else is required.
Why the corners are notched, not just cut
Fold the returns up without removing the corner squares and the material has nowhere to go: it bunches, the fold line pulls, and the fascia ripples. The notch is a through cut; the groove is a partial cut. Getting those two confused is the single most common ACM mistake.
The groove is on the back
Rout the V-groove into the reverse face and leave 0.2–0.5 mm of skin or core as the hinge. On 4 mm ACM that is a groove around 3.3 mm deep. Use a flat-tip bit: a sharp point touches the front skin and starts the crack you will see after the second fold.
Check the number before you cut
The two things worth checking on any flat pattern: the blank is bigger than the face by exactly 2R in both directions, and the groove rectangle is the finished size. If either is out, the pan will be out — and a 4×8 sheet is not free.
Questions
What is the flat pattern of an ACM panel?
The blank you cut: finished face plus twice the return depth in both directions, with an R x R square notched from each corner and one closed V-groove rectangle inset R on the back face.
How deep should the V-groove be?
60-70% of the panel thickness, leaving 0.2-0.5 mm as the hinge: about 2.4 mm on 3 mm ACM, 3.3 mm on 4 mm, 4.9 mm on 6 mm.
More: Rout and return, done properly · ACM pan signs · the full reference