TurboTax's calculations are correct. In this case, the maximum employer contribution is indeed $6,433.
This has to do with the fact that the employer contribution effectively reduces the amount of net earnings against which to compare the section 415(c)(1)(B) limit of 100% of compensation. That's why no more than 50% of what remains after subtracting employee contribution can be contributed as the employer contribution. It does not matter that the employee contribution is Roth. With the effective net earnings reduced to $29,933 by the $6,433 employer contribution, the amount available to contribute as an employer contribution is $29,933 - $23,500 = $6,433. Because the employer contribution must be subtracted from net earnings, the QBI is $29,933.
