You don't. If you back into a stopped vehicle, it is your fault. No if's, and's or but's, it's just 100% your fault.
If you don't have a clear view behind, you get out and check, or have someone 'spot' for you. That you honked your horn is irrelevant. It is NOT his duty to get out of the way, it's YOUR duty not to back into him.
Yes, there is law in NC about accidents on private property - it says that you are just as liable as you would be on the street.