The critical zc value for a 70% confidence interval is required. The values in a standard normal table that can be found in the link given below, gives the probability that a variable lies between 0 and x.
For a 70% confidence interval 0.35 lies to the left of the z = 0 and 0.35 lies to the right of z = 0.
Find the value of z that corresponds to 0.35 from the table.
This is approximately equal to 1.003.
The required critical values are approximately z = -1.003 and z = 1.003