Personally, no bleed on the seamed side (or at least have a clear, delineated mark for the seamed edge), and always measure both sides from the bottom, then flush the sides making up the seam. One seam won't be too bad, more than that and you can get in trouble. I also like to break the seams in the most convineint areas, even if that means I'll have a 7' and 9' panel instead of 2 8' panels, but this isn't always an option.
When you get one board laid, you butt the other against it with the boards aligned
Really? I used to make the biggest deal out of seams, worrying with aligning the previous panel to the next, but found that just sticking to measurements would provide the most consistent results.
(I use long super rulers with clamp on each end, holds everything firm, leaves no marks)
Oh, you'll like this. I use a 3" core, with 3" aluminum tube nestled inside. The aluminum gives it more rigidity, so when I put a squeeze clamp inside, it doesn't deform to round tube or buckle across the length of the tube. The cardboard makes it that much more forgiving when laying it on panels solo. clamp it down, flip the print over it, slit the backing and go to town.