update docs

docs/Dual_Carriage.md

@@ -28,6 +28,9 @@ relative to each other.
 Because Klipper configures this by the endstop position, you will
 need to calibrate your [rotation distance](Rotation_Distance.md) first.
 
+For the finest accuracy, you should consider calibrating the homing
+[endstop phase](endstop_phase.md) of your x axis motors. 
+
 ## Step 1 (~5mm)
 Start by literally eyeballing it. Put a piece of tape on your bed or
 use a line or other graphic already on your bed as a reference point.
@@ -59,7 +62,11 @@ Note the lines should be colinear. If they are not, adjust your
 `position_endstop` setting as noted in step 1. Once they are visually
 aligned, proceed to step 3.
 
-## Step 3 (~0.05mm)
+At this point you should re-tune your mins and maxes for each axis when
+the other is parked by issuing slow, small movements until they
+collide.
+
+## Step 3 (~.1 - .025mm)
 While the measuring process of this step accounts for symmetric
 horizontal expansion, you will likely achieve better results by
 calibrating the flow, temperature, and pressure advance for the
@@ -69,24 +76,49 @@ that can be printed without a heated bed.
 
 Use a slicer to generate g-code for the multi-part print found in
 [docs/prints/calibrate_idex.stl](prints/calibrate_idex.stl).
-Align the slot down the middle with the axis of your `dual_carriage`.
-Configure your slicer to print the 2 parts using extruders on opposite
-carriages. I highly recommend using identical filaments printed with
-the same temperature, etc. Nothing else matters as long as the wall
-quality is smooth. Print the object.
+Align the long direction of the print perpendicular to the axis of your 
+`dual_carriage`. Configure your slicer to print the upper part using 
+the right extruder. I highly recommend using identical filaments printed with
+the same temperature, etc. Use a rather coarse layer height since you
+have not yet calibrated your Z offset. Print the object.
+
+Use calipers or better, a micrometer to measure the 2 measureing points
+(refer to the image below). Complete the following table:
 
-Use calipers to measure the inner and outer distances as follows.
-Average your inner and outer measurements, calculate the difference
-from the nominal value (which is 40mm), and adjust your `position_endstop`
-setting as noted in step 1.
+| Part | Measurement | Calculation |
+|:--:|:--:|:--:|
+| Front | (mm) | 20 - (mm) |
+| Back  | (mm) | (mm) - 20 |
+| Offset |  | ^ Average ^ |
 
 ![measuring print](img/separation-block.png)
 
-# Next steps
-At this point you should re-tune your mins and maxes for each axis when
-the other is parked by issuing slow, small movements until they
-collide.
+You should be left with the adjustment that should be made to compensate
+for any alignment error. Save this to the printer by running:
+```
+SET_SEPARATION X_ADJUST={your calculated value}
+```
+If this sounds confusing, you can check your work with the following analysis:
+if the front measurement is more than 20mm by more than the difference of the back
+measurement and 20, the right extruder is printing too far to the right. The 
+`X_ADJUST` value should be negative because the offset needs to move to
+the left, towards the origin (or negative if your origin is centered).
+
+Repeat this step if nessesary until you are happy with the alignment and with the Y axis if nessesary. 
+
+## Step 4 (required for high quality printing)
+To print with low layer thicknesses, you'll probably find that the error
+between your nozzles in the Z axis is more than your layer height itself.
 
+Start with a piece of paper and switch between the extruders until the pressure feels roughly the same on both extruders. Make sure you're testing the exact same point on
+your bed. 
+
+Prepare to print a square about 10x10x5mm. Slice the object twice with the only difference being which extruder is used. MAKE SURE IT IS IN THE SAME PLACE ON YOUR BED. 
+
+Use calipers or better, a micrometer to measure the height of the two samples and
+adjust the Z offset as nessesary. 
+
+# Next steps
 Configure a homing override to make sure that the inactive carriage is
 fully parked in all scenarios.