Paint.NET :: Getting Correct Print Size?

[URL]

The 8x12 pixel size is 768x1152 and the 8x6 pixel size is 768x576. As to the need of being 11.5 I knew that, but I had this plan to work larger in order to downsize it and not lose any detail of clarity. If I'm understanding some of the other things I read you get a 100 pix per inch, but paint.nets is 96 per inch?

But let me throw a couple more things at you as well. I've made some scrapbook pages in what I had hoped were 10x13's to be printed out for framing...however, when taken to the Walmart printer it cuts off half of one side like the pic is too big..this is even when going for a 11x14 and larger size. I resized them, still cuts them off. Yet I make a 6x4 canvas and try to use it for my desktop background and it's too big for it in the "fill screen" mode, not mention the distortion.

Is there just a simple, cut and dried formula for image, canvas size that will come out the print dimensions I need? I'm sure I'm missing something basic, but my mind only works in basic mode!!