For and 8x10 design that is laid out at exactly 8x10 the scaling should be 100% vs. the 150%.
Think of scaling like zooming in or out. For instance if I am creating an 8x10 I create it on an 8.5x11 sheet and I layout the design in an 8x10 box centered in the middle. I also design away from the edges of that 8x10 to allow for matting. If I did my job right everything is exactly where it is supposed to be. Now if I go and print a PDF for example there are several scaling options in the page scaling box. I would suggest using none. The others may or may not shrink/expand your design when printed based on what math the PDF maker does to "correct the layout" or zoom in or out if you will.
Now positioning is a bit simpler. Depending on your program and printer you sometimes have options for moving the printed image around on the page (tiling, centered, fit) so be wary of those options as well.
It really boils down to three things, how you laid out your design, the programs you use, and any printer settings that your printer may throw into the mix.
Hope this makes sense.