The affix ṇamul comes after a root when a noun in the Instrumental -3rd case or Locative -7th case is in composition with it and when measure of length is intended.,
The word प्रमाणं means 'length or extension'. Thus द्वयङ्गुलोत्कर्षं ( द्वयङ्गुले उत्कर्षम् or द्वयङ्गुलेनोत्कर्षम्) खण्डिकां छिनत्ति 'he cuts pieces of the length of two fingers'. See 2.2.21.,