diff --git a/core/superslab/superslab_inline.h b/core/superslab/superslab_inline.h
index ad3305a8..54cd64b7 100644
--- a/core/superslab/superslab_inline.h
+++ b/core/superslab/superslab_inline.h
@@ -30,19 +30,33 @@ static inline uint8_t* tiny_slab_base_for(SuperSlab* ss, int slab_idx)
 }
 
 // Compute slab index for a pointer inside ss.
+// Box 5 wrapper: inverse of Box 3 geometry (tiny_slab_base_for_geometry).
+// Layout (data regions):
+//   - Slab 0: [ss + SUPERSLAB_SLAB0_DATA_OFFSET, ss + SLAB_SIZE)
+//   - Slab 1: [ss + 1*SLAB_SIZE, ss + 2*SLAB_SIZE)
+//   - Slab k: [ss + k*SLAB_SIZE, ss + (k+1)*SLAB_SIZE)
 static inline int slab_index_for(SuperSlab* ss, void* ptr)
 {
-    if (!ss || !ptr) return -1;
+    if (!ss || !ptr) {
+        return -1;
+    }
 
     uintptr_t base = (uintptr_t)ss;
     uintptr_t p    = (uintptr_t)ptr;
     size_t ss_size = (size_t)1 << ss->lg_size;
 
+    // Outside overall SuperSlab range
     if (p < base + SUPERSLAB_SLAB0_DATA_OFFSET || p >= base + ss_size) {
         return -1;
     }
 
-    size_t rel = p - (base + SUPERSLAB_SLAB0_DATA_OFFSET);
+    // Slab 0: from first data byte up to the end of first slab
+    if (p < base + SLAB_SIZE) {
+        return 0;
+    }
+
+    // Slabs 1+ use simple SLAB_SIZE spacing from SuperSlab base
+    size_t rel = p - base;
     int idx = (int)(rel / SLAB_SIZE);
     if (idx < 0 || idx >= SLABS_PER_SUPERSLAB_MAX) {
         return -1;