From your description I'm not sure what you are doing wrong, but TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet properly implements the Deduction Worksheet for Self-Employed in IRS Pub 560.
When you mark the Maximize box for an individual 401(k), are you leaving the blank the boxes for dollar amounts?
$67,500 is limit for the combined additions to your a 401(k), including employer contributions. If your actual additions total more than $67,500, you'll have an excess contribution subject to a 10% penalty on Form 5330 (which TurboTax does not support) and TurboTax will limit your self-employed retirement deduction to $67,500.
Ignore the information that TurboTax provides in step-by-step mode regarding amounts that you can contribute, that information is a mess and can be misleading. Look at the Keogh, SEP, and SIMPLE Contribution Worksheet itself.
