Planning my shed - Part 1

I was at Home Depot today looking at their sheds. I went inside one that was 8' x 12' - only 3 feet shorter than the Sonoma Shanty. Boy did it look small inside there! I'm glad I'll be building my Sonoma Shanty shed as a 'test bed' to get an appreciation of how much room is really inside a little house. The side walls were about 6' high in the HD shed and then the roof rafters began. The Sonoma Shanty side walls are 7' 5" high to allow extra room in the loft. For my shed, I plan to reduce the wall height to about 6' 5", since I don't need the extra height for a loft. This will make my shed 11' tall instead of 12'; I don't want my shed too tall in my backyard. My neighbor's shed is 10.5' tall, so I should be ok.

I also looked at exterior steel doors. They had a 36" x 78" six panel door for $120. I noticed that on the Sonoma Shanty plans, the rough opening height is 6' 4" for a 6' 8" door, which is incorrect. The 6' 6" door that I looked at today requires a RO height of 80.5". The reason I was looking at this, is if I reduce my wall height by a foot, I want to be sure I can fit in a standard door. I plan to put the door in one of the end walls, so it will fit. I will just raise one of the horizontal plates in the middle enough to fit a standard door in.

I looked at 3/8" Texture 1-11 for my exterior walls; it is going for $25.50 per sheet. I see it has shiplap edges on the long sides. What I'm not sure about is the triangular pieces on the top portion of each end - how do I seal the seam between the upper and lower pieces of T1-11?