You understate your tax if the tax shown on your return is less than the correct tax. The understatement is substantial if it is more than the larger of 10 percent of the correct tax or $5,000 for individuals. For corporations, the understatement is considered substantial if the tax shown on your return exceeds the lesser of 10 percent (or if greater, $10,000) or $10,000,000.
You may avoid the substantial understatement penalty if you have substantial authority for your tax treatment of the item or through adequate disclosure. To avoid the substantial understatement penalty by adequate disclosure, you must properly disclose the position on the tax return and there must at least be a reasonable basis for the position.
To properly disclose the position, complete and attach IRS Form 8275 to your tax return and disclose all relevant facts. A reasonable basis is a relatively high standard of tax reporting, that is, significantly higher than not frivolous or not patently improper. The position must be more than just merely arguable or merely a colorable claim. The position must be reasonably based on authority supporting the position.