I have used TurboTax the last few years.....do you need a certain level of the software to get the ability to do the conversion scenarios?
Nope, the way I do it is as follows:
For 2013 tax return
Start 2012 version with your 2012 tax return.
Save-as a new version, say "2013 Test"
Under Retirement plans, add a 1099-R and follow the step by step directions.
for boxes 1 and 2a enter the same amount, e.g. $50,000 and then box 2b should be checked (taxable amount not determined)
Box 7 should be code 2
check the box labeled IRA/SEP/SIMPLE
Click continue and then answer the questions saying that you converted it to a ROTH account.
You can then go to the forms view, select that 1099-R and simply enter different numbers for the withdrawal amounts in Box 1 and 2a, and see the amount of tax you owe. e.g. enter $1 and note the tax owed, then enter $50,000 and note the difference in tax owed.
It's only an estimate but as soon as the new version of TT is out you can get a much better estimate if it is out before year end.