How to find the least count vernier callipers and how to find the value of 1main scale division

Dear student ,
Least count is the minimum measurement that can be made by a measuring device.
Least count of a vernier caliper = minimum measurement on main scale / total number of divisions on vernier scale. Let's derive this:
Least count (L.C) of vernier calipers is the minimum length or thickness measurable with the
$Least count (L.C) =1 M.S.D -1 V.S.D$
$$\begin{gathered} L.C = 1 M.S.D - \frac{N-1}{N}*M.S.D \\ Since the V.S.D is \frac{N-1}{N} times the main scale division \end{gathered}$$
$$\begin{gathered} \Rightarrow L.C=1 M.S.D\left(1-\frac{N-1}{N}\right) \\ \Rightarrow L.C=\frac{1 M.S.D}{N}=\frac{S}{N} \end{gathered}$$
Where S is the value of 1 Main scale division and N is the number of equal divisions on the vernier scale.
Smaller is the magnitude of least count of a measuring instrument. more precise the measuring .

Now for the reading of main scale first let us discuss the main scale  .  there are two scale in the vernier calipers 
from these two the main scale is one which doesn't move and whose measurement is known to the observer . Like mainly an mm scale is used as main scale , so its measurements we know how to take . where the vernier's zero mark coincides with the main scale , this measurement on main scale is called main scale division .

Regards