"Typing" $300 in Line 10b doesn't work.  Nothing typed onto the Form 1040 line 10b enters.  You can override the blank space in line 10b, BUT line 10c does not do the subtraction of the $300..

Let's face it, Intuit has not finished the programming for line 10b.

This affects a lot of returns where the taxpayer(s) take a standard deduction.