diff --git a/wizard-work/beam/test_token_bucket.py b/wizard-work/beam/test_token_bucket.py
index 1612d8e..3e54748 100644
--- a/wizard-work/beam/test_token_bucket.py
+++ b/wizard-work/beam/test_token_bucket.py
@@ -21,12 +21,20 @@ def test_capacity_cap():
 
 def test_acquire_blocks_and_returns():
     b = TokenBucket(rate=20.0, capacity=1.0)
-    b.try_acquire(1.0)  # drain
+    # t0 must precede the drain: try_acquire() debits the last token and sets
+    # the refill origin (self._last) inside the call, so a t0 taken after it
+    # silently subtracts whatever the scheduler spent in between.  Observed at
+    # 7c36185 as `assert 0.03909331373870373 >= 0.04` in a loaded full-suite
+    # run: ~10.9 ms of scheduler gap, i.e. 0.05 s of withheld tokens minus a
+    # prefix the cell never measured.  dt from before the drain is >= 0.05 by
+    # construction (acquire cannot return until the refill origin is 0.05 s
+    # old), so the 0.04 floor has margin instead of sitting 1 ms from the edge.
     t0 = time.monotonic()
+    b.try_acquire(1.0)  # drain; sets the refill origin
     ok = b.acquire(1.0, timeout=0.2)
     dt = time.monotonic() - t0
     assert ok is True
-    assert dt >= 0.04  # had to wait ~0.05s for 1 token at 20/s
+    assert dt >= 0.04  # 1 token at 20/s = 0.05s withheld, measured from the origin
 
 
 def test_n_exceeds_capacity_raises():