How do you calculate least count of spectrometer?

The value of each main scale divisions is half a degree(30') .

The number of vernier scale divisions is 30.

29 main scale divisions are divided into 30 vernier scale divisions (30 VSD = 29 MSD )

Therefore value of 1 VSD = 29/30 MSD.

Least Count (L.C.) = 1MSD - 1VSD

= 1MSD - 29/30 MSD

= 1/30 MSD

= 1/30 x 30'

(L.C.) = 1' (1 minute)