I would ask your question on the John Bridge Tile forum. The pros there should be able to give you good advice.

Advice?

Plan your layout carefully, starting from the center point in each run, and arranging your layout so that you don't have any tiny pieces on the ends. In the center, you can either have a full tile or a seam between two full tiles -- whichever works out better on the edges. If you have accent tiles, check to be sure their planned locations aren't right where your outlets are. (They almost always are!)

Plan your edges so they don't end with a cut tile. Bullnoses or trim pieces usually make the best edges, with the 'cut to fit' cuts either on the inside of the trim tile, or on the outside edge of the last field tile before the trim piece.

Measure twice, cut once. And never assume your walls are flat, your verticals plumb, or your countertops level.

Draw grid lines, plumb lines and level lines to keep your layout on track.

Most times, everything will not be perfect. So decide what the most critical areas are (not to be short-changed) and where any 'slop' should go for minimal visibility.

Take your time and work as slowly as you need to keep things perfect. Yes, the work will take you three times as long as it would take a professional. BUT - If you take that kind of care, your work will look better than the average 'professional' work. (Bill's not an 'average' tile pro!)

Yes - You can drive yourself crazy. But backsplashes are small, so 'crazy' won't last forever.

A couple of things. First off, to determine just how accurate the sizing is, lay a bunch of tiles dry on the countertop, or some other flat surface. Lay them out in say, a square, about 24x24 (and lay them out all lining up, rather than the usual brick pattern), and butt them up tight together. If they're going to "jump" at all, you'll see it, and you can then determine just how much you need to open them up to avoid any problems.

As for plumb and level grid lines, with a brick joint, you might need a couple of lines to keep you "honest" down the backsplash, but you're not dealing with that big an area, or that intricate a pattern where you need to grid out the wall. The biggest thing you want to check is once you decide your joint size, lay out 6 tiles with the exact joint size, and check the measurement of those 6 tiles to the measurement you have from the countertop to the bottom of the cabinets. Depending on the exact size of the tiles, and exactly how far apart your upper and lower cabinets are spaced, that very well could be a full tile. It could also mean you have a 1/2" gap at the top, meaning you'd have to start with a cut tile at the bottom of your backsplash.

I think that's enough to confuse you for now. :-) Once you've determined that much, then we can take it from there.