You are asking about the natural abundance of the various isotopes of lithium. All of the elements found in nature exist in several different isotopes. Isotopes of an element differ in the number of neutrons in the nucleus. Each isotope of an atom will exist in varying ratios in nature. The weighted average of each isotope is used for the average atomic weight for each element as found on the periodic table. But there is no calculation for the natural abundances of any isotope. The isotopes exist as they naturally do and all scientists can do is measure their relative abundances accordingly.
Lithium naturally exists in two different isotopes, Li-6 and Li-7. Li-7 makes up 92.4% of naturally occurring lithium and Li-6 makes up the other 7.6%. Other isotopes of lithium do exist, but they are all unstable and do not have a long lifetime of existence. The average atomic weight of lithium is 6.941.