On my OMB 1099R 2a shows Taxable Amount UNKNOWN. How much tax should I pay on this ?

[xmasbaby0]

The software will calculate the tax owed--if any.   "unknown" is there because the financial institution that issued the 1099R does not have any way to know what other taxable income you received.   

[spuk00]

You will use the simplified method.  When entering your 1099-R, next to the taxable amount is a learn more link.  click on that, scroll down until you find the simplified method link. Click on that and follow the instructions.