It's not stored in custom header, it's stored in Nintendo OPUS header which belongs to type 1. I don't understand what custom header in type 1 represents at offsets 0x10, 0x14, 0x18, 0x1C, and 0x20, so I can't help make converter.
type 1 header looks like this:
0x0 int32: 0x00000001
0x4 int32...