Manual of the Freiburg Vision Test ‘ FrACT

FrACT unfortunately has too often been used without calibration. Thus from version 3.2 onwards a strong warning message appears on every start unless you have changed both the observer distance entry and the length of the blue calibration ruler (see preceding paragraph). Both are defaulted to numbers that are unlikely to occur in practice (399 cm and 169 mm). As long as one of these remains at the default value, they are highlighted in red and that obnoxious message appears upon every start of FrACT. 
 Red left rectangle:
 the length of the blue ruler line, measured in mm.