No Zone 2, Factor Zone 1 BCTR by 2/3?

I am editing this question to hopefully make it more clear. Our project is hoping to achieve Bird Collision Deterrence. We have a question on the spreadsheet calculations. If the entire building facade is classified as Zone 1 (it is a single story building), when using the BCTR Spreadsheet should we use the Z1 BCTR output as the Total Building BCTR or the Total Building BCTR calculation? The Z2 BCTR does not work when its material area is set to 0, which in turn means the Total Building BCTR formula does not work. However, if I manually set the Z2 BCTR to 0 in the Total Building BCTR formula, it does work, and effectively reduces the Z1 BCTR by 1/3 to reach the Total BCTR. Which is the correct number to use?