Just to be clear.  You do enter and report the full SSA-1099 box 5 on your tax return.  You don subtract any amounts from it.

You just don't enter the Medicare on the same input screen.    Medicare plan B payments are qualified as Self-employed medical insurance premiums and should be entered under Business instead of in the Social Security Benefits entry area.

And the health ins deduction does not actually show up on schedule C.  Self-employed health insurance deduction goes on Form 1040 Schedule 1 line 29, as long as the expense is not greater than your net self-employment income. If it does exceed your net self-employment income it gets split automatically. An amount equal to your net self-employment income goes on Form 1040 Schedule 1 line 29, and the remainder gets added in to medical expenses on Schedule A.