I've finished entering everything; actually I am ready to file my return barring this bizarre situation.
Below are more data points. seems TT is using a rate of 1% (0.6% - 1.5%).
- $914 => $1,399
- $1,914 => $1,404
- $2,914 => $1,410
- $3,914 => $1,422
- $4,914 => $1,427
- $5,914 => $1,439
Further observations
- TT does not calculate it as 'box 1 interest', as my State income margin rate is much higher than 1%
- The Fed tax is accurately changed based my Fed income margin rate while tweaking the numbers
