diff --git a/Custom-M111-Full-Engine-Harness.md b/Custom-M111-Full-Engine-Harness.md
index 748d0791..57f841aa 100644
--- a/Custom-M111-Full-Engine-Harness.md
+++ b/Custom-M111-Full-Engine-Harness.md
@@ -1,6 +1,12 @@
 # Notes on fully custom M111 Engine Harness
 
+## Reference point
+
+While harness design is work in progress we use reference point R. R is currently 100cm from ECU.
+
+R+5 would mean "5cm further away from ECU from point R", "R-5" would mean "5cm closer to ECU"
+
 ## Low current subassembly
 
-x4 sensor gnd crimp
-x3 
+x4 sensor gnd crimp, R+40cm
+x3 sensor +5v crimp, R+40cm