Recently we added Index Scaling Factor, which modifies the interest rate and spread of tax-exempt loans by a percentage. If enabled, you will see the Scaling Factor fields on the Opportunity Screen when you set a loan as Tax Exempt. You can choose whether to apply the Scaling Factor to the Index Only or to the Index and Spread.
Index Scaling Factor will be disabled by default. If you have administrative access you can enable it for a product in the Product Edit Screen.
For more information about Index Scaling Factor, visit Index Scaling Factor.