It depends. You need to start with any stock you originally were gifted for 3M stock, which was the cost in the hands of the giver. Once it became yours, you would add any reinvested dividends to the original cost, as well as all shares and fractions of shares. This cost basis now becomes your stock basis for Solventum. Divide that cost by the number of shares received in Solventum on the day of the spin off.
From that point forward you will add any reinvested dividends for each share or fraction of shares you receive. Cost basis must be tracked until each stock purchase is sold.
Stock sales are first in-first out unless you specifically choose shares to sell. This means the cost will start at the beginning and an average cost basis is not allowed except for mutual funds.
Stock prices can be found on various websites and various times so it is possible to find historical stock prices as long as you have your purchase dates.
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