Depending on the size of your window cutout, you should have about 41 sq. ft of interior tank space to seal. So maybe 45 sq ft with top braces??
If so, three gallons will give you around 100 mil wet film thickness. I don't have an exact spec on how much the thickness of Pond Coat diminishes once the carriers evaporate but my educated guess is you'll be around the minimum mil thickness the company recommends.
Do an exact calculation of interior tank space factoring the front and top cutouts and email the company. That's always the best idea.
Short answer is three gallons minimum. But let us know what the company says.
