Modern tests of intelligence do not use the IQ formula developed by William Stern. How do modern tests of intelligence determine the standard for "normal" intelligence is?

Modern IQ tests run off of a statistically normalized distribution. 100 is normal intelligence based on the average intelligence of the group. It also uses a spread of 15 per standard deviation (e. g. if someone is 2 standard deviations above the average, they have an IQ of 100 + 15*2=130; similarly, if someone is 2 standard deviations below the average, they have an IQ of 100 - 15*2=70).