
Simply put, there is NO IUPAC rule based on summing the locants despite what you may have been taught elsewhere!
However, to be honest, the sum idea works in most cases (unfortunately). But, learning the "sum rule" typically means one doesn't take the time to learn the first point of difference rule which is required in several other situations. Therefore, it is recommended that you learn the first point of difference rule as you will probably need to learn it at some point anyway, and it really isn't any more difficult to learn!
(summing a list of numbers versus comparing a list of numbers written in numerical order).
HINT:
Here are some examples of the application of the first point of difference rule to generate the correct name where the "sum rule" fails:
Consider the following example (from an IUPAC nomenclature website):
If we were to sum, then the total from the first numbering scheme (left to right) = 16, the second scheme (right to left) = 17 which means if the sum rule existed, it would make scheme 1 the answer. However, let's follow a method for identifying the first point of difference... list out the locant set for each scenario in numerical order. Then compare the lists looking for the first time the numbers are different:

Consider the following example (from Carey "Organic Chemistry" by McGrawHill):
If we were to sum, then the total from the first numbering scheme (left to right) = 22, the second scheme (right to left) = 23 which means if the sum rule existed, it would make scheme 1 the answer. However, let's follow a method for identifying the first point of difference... list out the locant set for each scenario in numerical order. Then compare the lists looking for the first time the numbers are different:

©Dr. Ian Hunt, Department of Chemistry 