My tax spreadsheet uses the percent of AGI as described in the instructions. The calculation has not actually come into play with AGI minus the specified threshold resulting in the lower tax.
In regards to the tax deduction, the instructions state that you can allocate state, local, and foreign income taxes if properly deducted on your return. Your proposal does not appear consistent with that requirement.
I also wonder if the default method is entirely correct since one might be deducting some taxes related to the previous tax year that were paid in the current tax year. As I recall a similar issue has come up with deductions related to the Foreign Tax Credit.