should I include after-tax 401k assets converted to Roth then subsequently rolled over into my Roth IRA when tracking basis for the Roth IRA?

A rollover from a designated Roth account in the 401(k) to a Roth IRA, if done by direct rollover, would be reported on a code-H  Form 1099-R.  This addition to your basis in your Roth IRAs should automatically be tracked by TurboTax when you enter this Form 1099-R into TurboTax.  Examine TurboTax's IRA Information Worksheet after entering this Form 1099-R.