Waist-height cover gets vaulted. Here is the layering that does not.
Most perimeters fail the same way: they are built out of waist-height blocks, and infantry simply vault them.
Hesco Block (Small) and Sandbag Wall are one block tall. Cover to shoot over, and a step to climb. Hesco Block (Tall) and Hesco Wall (Quad) are two, which is full-body cover. The Bremer Wall is three, and topped with barbed wire.
Nothing can be built on top of a Bremer Wall. That makes it the finishing layer, not a foundation. Run tall Hesco for the wall, then cap it with Bremer, and the result cannot be climbed without taking damage.
Sandbags are designed to sit on top of Hesco. Low Hesco with sandbags stacked on it is the combination the community settled on for CIWS emplacements. High enough to protect the gunner, low enough not to block the gun.
The planner counts how much of your cover is still waist height, under Anti-climb, so you can see the weakness before somebody finds it.