If a person is not sure how many fasts were missed, how should he pay the fidyah?

If a person is not sure how many fasts were missed, how should he pay the fidyah?

The individual should make a reasonable estimate and pay thefidyah based on that estimate. 
Reference: Al-Nawawi, al-Majmu’ Syarh al-Muhadzdzab, Vol. 6, p. 271.