A method for determining a set point for measurements from a temperature sensor of an apparatus for the layer-by-layer formation of a three-dimensional object from particulate material, and associated controllers. The method includes distributing a calibration layer of particulate material over a build bed surface; selectively applying absorption-modifying fluid to a reference area or a surrounding area thereof, on the build bed surface; (c) lowering the build bed surface to a calibration depth, (d) applying heat to the reference area using a moveable heat source while measuring the temperature increase of a sub-reference area over a duration of time and/or taking optical readings of an optical property of the sub-reference area over the duration of time; (e) determining the onset of fusion of the particulate material; and (f) applying the onset of fusion as the set point for subsequent temperature measurements.