The 2011 manual gives two thicknesses. It defines casing as a 3 to 4 cm layer, then instructs an even 4 to 5 cm applied using 4 cm iron rings or wooden blocks. The Hindi farmer folder says 3 to 4 cm. Pick one figure, write it down and keep it the same on every bed.
Evenness matters more than the number itself. A thin patch dries first and pins early, a thick patch pins late, and the bed then carries two flushes at once. Do not press the layer down, and water it lightly rather than flooding it.