diff --git a/geomagio/residual/Calculation.py b/geomagio/residual/Calculation.py
index a6e942c82dbdc7bde889f14a8c80b8eb2001fe80..e5ef4e23e7c8ad2388744978da6121305fbcf653 100644
--- a/geomagio/residual/Calculation.py
+++ b/geomagio/residual/Calculation.py
@@ -161,8 +161,8 @@ def calculate_D(ordinates_index, measurements, measurements_index, azimuth, h_b)
     # add 100 if mark up is greater than mark down
     # subtract 100 otherwise
     if (
-        measurements_index["FirstMarkUp"][0].angle
-        < measurements_index["FirstMarkDown"][0].angle
+        measurements_index[mt.FIRST_MARK_UP][0].angle
+        < measurements_index[mt.FIRST_MARK_DOWN][0].angle
     ):
         average_mark += 100
     else: