This is a calculation that TurboTax is incapable of handling directly. Since you've already maxed out your elective deferral at your W-2 employer, you can cause TurboTax to calculate only the employer contribution you can make to your individual IRA by marking the Maximize box for a SEP-IRA contribution or a Profit Sharing Keogh contribution instead of making any entries on the Individual 401(k) page.
