If your company did a 10-for-1 stock split, you should have ten times the number of shares you originally received. However, your original per share cost basis needs to be adjusted to reflect the fact that you have ten times the number of original shares. Thus, divide your original per share cost basis by ten, which will give you the post split per share cost basis. The amount of the original cost basis does not change in a stock split, but what does change in the per share cost basis.
Your original cost basis is the value of the shares on the day they vested. So if you sold some shares, you should calculate the per share cost basis.
Fidelity probably won't know your cost basis, so you will have to enter that information. Sometimes, firms will include a zero as the cost basis, and that is why we inform taxpayers to adjust their cost basis if necessary otherwise they will owe more tax than is required.
@murreweet
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