The Congressional Budget Office has released Computing Effective Tax Rates on Capital Income (110 pages).  Here is part of the Preface:

This CBO background paper explains and documents the effective tax rate calculations presented in the October 2005 CBO paper Taxing Capital Income: Effective Rates and Approaches to Reform. It begins by presenting the formulas used to compute effective tax rates and then describes the data used to implement the formulas. An appendix provides a user’s guide to an Excel workbook that can be used to compute the effective tax rates in Taxing Capital Income.