The basis on your 8606 goes against your traditional / SEP IRA distributions. For example: you have $10,000 in a traditional IRA, it is normally all taxable. If you have an 8606 with $7,000 basis, then only $3,000 is taxable.
When you switched to a ROTH, the income would have been the $3,000.
If you suffered losses, you may have a basis larger than the account value. You also moved parts and pieces so that values had to be proportionate. In the example, the basis was 70% of the value. I presume you had multiple accounts covered and moved.
You can amend up to 3 years returns - if you feel there was an error.
Reference:
About Form 8606, Nondeductible IRAs - IRS
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