Answer
(Fatwa: 1044/774/L=1431)
It is necessary to cut the hair equal to the tip of finger. If the hairs are too short to cut equal to one finger tip, then it is wajib to shave the head.
قال فى مجمع الانهر: ثم يحلق وهو أفضل من تقصير أو يقصر بأن يأخذ من شعره قدر الأنملة وجوباً و تقصير الكل مندوب والربع واجب (مجمع الانهر: 1/280)
So if your hair were too short to cut equal to finger tip then as per the question mentioned above, dam (slaughter of an animal) shall be wajib on you.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband