The Formula that the IRS uses is from
Internal Revenue Manual 25.18.5
| Injured Spouse's Separate Tax Liability | ÷ | Total of Spouses' Separate Tax Liabilities | × | Joint Tax Liability Shown on Return | = | Injured Spouse's Share of Tax Liability |
Separate Tax Liabilities are calculated using Married Filing Separate Tax Tables for each spouse as mentioned earlier. The form 8379 has instructions for breaking out the amounts for each spouse to be used in this calculation.
