Genghis Bob's supa-sekrit Border Wall calculation:
1. Click on "Collect Taxes"
2. On a scrap of paper, write down the "Total Collection Amount"
3. Click on "Improvements"
4. Buy a Border Wall
5. Repeat steps 1 and 2
6. Is your new number larger?
a. Yes, go to step 3
b. No, destroy a Border Wall and stop
I know it's not sexy, but it works.