tuning_tower: compensate z-offset (#2881)

Signed-off-by: Rubens Panfili <rubens.panfili@gmail.com>

klippy/extras/tuning_tower.py

@@ -45,6 +45,10 @@ class TuningTower:
         self.last_position = list(pos)
         return pos
     def calc_value(self, z):
+        # Compensate Z-Offset
+        gcode = self.printer.lookup_object("gcode")
+        z_offset = gcode.get_status(None)['base_zpos']
+        z = max(0, z - z_offset)
         if self.band:
             z = (math.floor(z / self.band) + .5) * self.band
         return self.start + z * self.factor