diff --git a/tests/AcDream.Core.Tests/Terrain/TerrainModernConformanceTests.cs b/tests/AcDream.Core.Tests/Terrain/TerrainModernConformanceTests.cs
index 3bc403b..c02f7cc 100644
--- a/tests/AcDream.Core.Tests/Terrain/TerrainModernConformanceTests.cs
+++ b/tests/AcDream.Core.Tests/Terrain/TerrainModernConformanceTests.cs
@@ -41,7 +41,7 @@ public class TerrainModernConformanceTests
         ("Mid-map 0x7F7F",           0x7F, 0x7F),
         ("MapCorner 0xFEFE",         0xFE, 0xFE),
         ("Subway outdoor 0x0185",    0x01, 0x85),
-        ("North continent 0x4D96",   0x4D, 0x96),
+        ("North continent 0x4D96",   0x4D, 0x96),  // worst-case landblock from SplitFormulaDivergenceTest
     };
 
     [Fact]
@@ -102,14 +102,19 @@ public class TerrainModernConformanceTests
             var surfaceCache = new Dictionary<uint, SurfaceInfo>();
             var meshData = LandblockMesh.Build(landblock, lbX, lbY, heightTable, ctx, surfaceCache);
 
-            // Sample 100 (localX, localY) points uniformly in [0, 192).
-            // We avoid the exact upper bound (192) because that maps to
-            // cell index 8 which the physics path clamps; the pure mesh
-            // sampler doesn't have triangles past 192 anyway.
+            // Sample 100 (localX, localY) points uniformly in [0, 191.975].
+            // The physics path clamps fx = localX/24 to (CellsPerSide - 0.001f)
+            // = 7.999, which corresponds to localX <= 7.999 * 24 = 191.976.
+            // Sampling beyond that boundary makes physics compute Z at the
+            // clamped position while the mesh sampler uses the actual
+            // position — a difference of up to 23 mm at the upper edge,
+            // which on a steep slope would falsely trip the 1 mm sentinel.
+            // Stay strictly below the clamp boundary so both oracles
+            // compute Z at the same (cellX, tx).
             for (int s = 0; s < 100; s++)
             {
-                float lx = (float)rng.NextDouble() * 191.999f;
-                float ly = (float)rng.NextDouble() * 191.999f;
+                float lx = (float)rng.NextDouble() * 191.975f;
+                float ly = (float)rng.NextDouble() * 191.975f;
 
                 float meshZ = SampleMeshZ(meshData, lx, ly);
                 float physicsZ = TerrainSurface.SampleZFromHeightmap(