"Also, I have a very small traditional IRA (with post tax contributions) but would not expect that to effect the amount." - yes it does.
For tax purposes you only have ONE Traditional IRA which can be split between as many different accounts as you want, but for tax purposes they are all added together. Any after-tax basis applies to any distribution from any Traditional IRA account.
TurboTax will ask for your non-deductible "basis" and then the *Total Value* of *all* Traditional IRA accounts as of Dec 31, of the tax year. That is so the prorating of the basis can be properly proportioned between the current years distribution and the remaining IRA value. That is done on the 8606 form.