How do I find z0 and z1, when the smallest value for phi(z) on my Normal Distribution Lookup Table is 0.5000 ?

I recall that my teacher explained a method of doing this a few months back, but cannot seem to remember it..

My guess is something like linking the small value to a larger one which is on the table, and then reversing it later on, perhaps something like this?..

Is phi the normal curve or the integral of the normal curve?

The normal curve is symmetric around its mean.

If phi(0) = 1.0 then it's the normal curve and phi(z) = phi(-z)

If phi(0) = 0.5 then it's the integral of the normal curve and phi(-z) = 1 - phi(z)