To determine if a value is accurate compare it to the accepted value. As these values can be anything a concept called percent error has been developed. Find the difference (subtract) between the accepted value and the experimental value, then divide by the accepted value.
% error = (accepted - experimental) / accepted *100%
To determine if a value is precise find the average of your data, then subtract each measurement from it. This gives you a table of deviations. Then average the deviations. This will give you a value called uncertainty. A plus or minus value that says how precise a measurement is.
deviation = (average - actual)
standard deviation = (deviations for all measurements added together) / number of measurements
Vernier Constant
It is the ratio of smallest division of main scale to the number of divisions of vernier scale.