The entry would be 0.5 for 50% What do you think this will do? Press F1 at that entry point and you get::

(9a) Total Distribution Percent (.xxxx or 1 for 100 Percent)

Enter the distribution percentage shown on Form 1099-R box 9a. According to Form 1099-R instructions, if this is a total distribution and it is made to more than one person, the percentage in this box represents the percentage received by the person whose name appears on the Form 1099-R.   The percentage is used to compute ordinary income, death benefit exclusion, current actuarial value of annuity, federal estate tax, and the tax calculation per the worksheet in the Form 4972 instructions. Refer to the ‘Form 4972’ section of input on this screen for additional required entries.