I think that the "reserve" starts when your gauge hits 1.0. I have repeatedly run my tank down to 0.0 and it usually takes 8.5 when I do this. Recently there was one exception where I ran to 0.0 and then drove another 10 miles or so. I filled the tank with 8.9, but I question the accuracy of the pumps at the particular station I was at. I don't go there often, but every time I do, it always shows that it fills a little more that I think it should actually be.
So yes, the 1.3 reserve is a part of the 8.7 gallon tank. The service manager at my dealership has confirmed this as well, and I tend to trust him.