diff --git a/src/mx_bluesky/hyperion/experiment_plans/change_aperture_then_move_plan.py b/src/mx_bluesky/hyperion/experiment_plans/change_aperture_then_move_plan.py
index 69e845077..930f4428b 100644
--- a/src/mx_bluesky/hyperion/experiment_plans/change_aperture_then_move_plan.py
+++ b/src/mx_bluesky/hyperion/experiment_plans/change_aperture_then_move_plan.py
@@ -26,8 +26,9 @@ def change_aperture_then_move_to_xtal(
             best_hit.bounding_box_mm[1] - best_hit.bounding_box_mm[0]
         )
         with TRACER.start_span("change_aperture"):
-            yield from _set_aperture_for_bbox_mm(
-                aperture_scatterguard, bounding_box_size
+            yield from set_aperture_for_bbox_mm(
+                aperture_scatterguard,
+                bounding_box_size,
             )
     else:
         LOGGER.warning("No bounding box size received")
@@ -49,25 +50,35 @@ def change_aperture_then_move_to_xtal(
         )
 
 
-def _set_aperture_for_bbox_mm(
-    aperture_device: ApertureScatterguard, bbox_size_mm: list[float] | numpy.ndarray
+def set_aperture_for_bbox_mm(
+    aperture_device: ApertureScatterguard,
+    bbox_size_mm: list[float] | numpy.ndarray,
 ):
-    # TODO confirm correction factor see https://github.com/DiamondLightSource/mx-bluesky/issues/618
-    ASSUMED_BOX_SIZE_MM = 0.020
-    bbox_size_boxes = [round(mm / ASSUMED_BOX_SIZE_MM) for mm in bbox_size_mm]
-    yield from set_aperture_for_bbox_size(aperture_device, bbox_size_boxes)
+    """Sets aperture size based on bbox_size.
 
+    This function determines the aperture size needed to accomodate the bounding box
+    of a crystal. The x-axis length of the bounding box is used, setting the aperture
+    to Medium if this is less than 50um, and Large otherwise.
+
+    Args:
+        aperture_device: The aperture scatter gaurd device we are controlling.
+        bbox_size_mm: The [x,y,z] lengths, in mm, of a bounding box
+        containing a crystal. This describes (in no particular order):
+        * The maximum width a crystal occupies
+        * The maximum height a crystal occupies
+        * The maximum depth a crystal occupies
+        constructing a three dimensional cuboid, completely encapsulating the crystal.
+
+    Yields:
+        Iterator[MsgGenerator]
+    """
 
-def set_aperture_for_bbox_size(
-    aperture_device: ApertureScatterguard,
-    bbox_size: list[int] | numpy.ndarray,
-):
     # bbox_size is [x,y,z], for i03 we only care about x
     new_selected_aperture = (
-        ApertureValue.MEDIUM if bbox_size[0] < 2 else ApertureValue.LARGE
+        ApertureValue.MEDIUM if bbox_size_mm[0] < 0.05 else ApertureValue.LARGE
     )
     LOGGER.info(
-        f"Setting aperture to {new_selected_aperture} based on bounding box size {bbox_size}."
+        f"Setting aperture to {new_selected_aperture} based on bounding box size {bbox_size_mm}."
     )
 
     @bpp.set_run_key_decorator("change_aperture")
diff --git a/tests/system_tests/hyperion/test_aperturescatterguard_system.py b/tests/system_tests/hyperion/test_aperturescatterguard_system.py
index b92499b03..c05a82ce8 100644
--- a/tests/system_tests/hyperion/test_aperturescatterguard_system.py
+++ b/tests/system_tests/hyperion/test_aperturescatterguard_system.py
@@ -11,7 +11,7 @@
 from ophyd_async.core import DeviceCollector
 
 from mx_bluesky.hyperion.experiment_plans.change_aperture_then_move_plan import (
-    set_aperture_for_bbox_size,
+    set_aperture_for_bbox_mm,
 )
 
 
@@ -29,7 +29,17 @@ def ap_sg():
 
 
 @pytest.mark.s03()
-def test_aperture_change_callback(ap_sg: ApertureScatterguard):
+@pytest.mark.parametrize(
+    "bbox, expected_aperture",
+    [
+        ([0.05, 0.05, 0.05], "LARGE_APERTURE"),
+        ([0.02, 0.02, 0.02], "MEDIUM_APERTURE"),
+    ],
+    ids=["large_aperture", "medium_aperture"],
+)
+def test_aperture_change_callback(
+    ap_sg: ApertureScatterguard, bbox: list[float], expected_aperture: str
+):
     from bluesky.run_engine import RunEngine
 
     from mx_bluesky.hyperion.external_interaction.callbacks.aperture_change_callback import (
@@ -39,5 +49,5 @@ def test_aperture_change_callback(ap_sg: ApertureScatterguard):
     cb = ApertureChangeCallback()
     RE = RunEngine({})
     RE.subscribe(cb)
-    RE(set_aperture_for_bbox_size(ap_sg, [2, 2, 2]))
-    assert cb.last_selected_aperture == "LARGE_APERTURE"
+    RE(set_aperture_for_bbox_mm(ap_sg, bbox))
+    assert cb.last_selected_aperture == expected_aperture