Break-Even Tax Rate
The future marginal tax rate at which converting to Roth today and leaving the money pre-tax end up equal. If you expect a higher rate later than the BETR, converting wins.
When conversion tax is paid from the converted funds, the BETR equals the rate paid today. Paying the tax from a taxable account lowers the BETR, because it moves more money under the Roth's tax shelter.
RetIQ reports the BETR in the Roth Conversion Impact Analysis.