Use the correct fill factor for splits.

btree.sql

@@ -292,12 +292,12 @@ CREATE FUNCTION insert(
             n.ks := n.ks[1:n.ix-1] || k || n.ks[n.ix:n.nk];
             n    := reset(n);
 
-            IF n.nk < 8 THEN
+            IF n.nk < m.fill THEN
                 PERFORM update(n);
             ELSE
-                s.ks    := n.ks[1:4];
+                s.ks    := n.ks[1:m.fill/2];
                 s.leaf  := n.leaf;
-                n.ks    := n.ks[5:8];
+                n.ks    := n.ks[(m.fill/2 + 1):m.fill];
 
                 s := update(s);
                 n := update(n);

t/btree.pg

@@ -127,5 +127,22 @@ SELECT btree_is(11, 8, $$
     (11,    '{10,17,1}',        false)
 $$,                         'overflow splits rather than stealing');
 
+SELECT reset_btree(256);
+SELECT btree_is(1, 256, $$
+    (1,     '{}',               true)
+$$,                         'initial btree (order 256)');
+
+SELECT perform(insert(null)) FROM generate_series(1,255);
+SELECT bag_eq(
+    $$ SELECT id FROM btree $$,
+    $$ VALUES (0), (1) $$,
+    'order-256 btree will take 255 values without splitting');
+
+SELECT perform(insert(null));
+SELECT bag_eq(
+    $$ SELECT id FROM btree $$,
+    $$ VALUES (0), (1), (258), (259) $$,
+    'order-256 btree splits at 256 entries');
+
 SELECT * FROM finish();
 ROLLBACK;