diff --git a/devtimer.py b/devtimer.py
index 45276ef..0b63f7c 100755
--- a/devtimer.py
+++ b/devtimer.py
@@ -26,7 +26,7 @@
 DEBUG = True                  # Debugging switch
 
 BEEP = 15                     # Beep interval
-CALIBRATION_OFFSET = 0.002    # Compensate for program overhead in master loop
+CALIBRATION_OFFSET = 0.003    # Compensate for program overhead in master loop
 
 # Profile Constants
 
@@ -35,8 +35,12 @@
 FILM = 2
 
 
-# Globals (So shoot me ...)
-# These get updated by the thereads reading the switches and thermocouple
+# Globals
+
+# These get updated by the threads that read the switches and
+# thermocouple.  On a slow machine like the Pi Zero, we want to avoid
+# unnecessary function calls, so we make these globally RW.
+# So, shoot me ...
 
 CURRENT_PROFILE = FILM
 CURRENT_TEMP = 20        # Stored as index relative to 60F
@@ -52,7 +56,13 @@
       Paper
       Film
 
-  Each contains entires   for muliplicative corrections from 60F to 80F.
+  Each contains entires for multiplicative corrections from 60F to 80F.
+
+  The profile global above selects which of these tuples to index into
+  - using the normalized temp global above as the index.  We don't
+  want to use a dictionary here (with profile as the key) because of
+  the overhead that incurs.  Straight tuple indexing should be much
+  quicker.
 
   WARNING: It takes about 250ms to update the display on a Pi Zero.
            So, if the "virtual second" falls at or below this, the