The LOOKUP¨function looks for the value TODAY()-M2 ( length of service) in the first range ( callled Lookup_vector).
If not found it looks for the first number larger than said value, and returns the value with the same position in the second range( called result_vector).
For this to work values in the lookup_vector must be placed in ascending order.

This link will also help.

An INDEX/MATCH combination might also work

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