Question

if the screw is with no zero error then what formula we should use

Answer 1

if the vernier calliper also has no zero error then formula for finding the lenth

Answer 2

well you should specify formula for what?we can measure diameter of wire ,volume of glass plate etc by screw gauge..but anyways your main concern seems to be zero error.there are two types of zero errors positive and negative ,if positive zero error is there correction is negative i.e you will subtract that with observed head scale reading .if negative zero error correction is positive i.e you will add that in observed head scale reading bu if no zero error as in your case no correction is needed you will directly write your observed head scale reading. it depends on what you are measuring you will proceed further