Okay, I think I figured it out. Rather than investing in very expensive flooring estimator software, we can use Home Designer to display and manipulate tile placement. You can create in Photoshop or import a .grid in png or .gif to do your work. First, count the squares in both directions. Secondly, multiply each square times the size, e.g. 12 inches and add the grout spacing for each tile. If the grid is 40 squares wide by 32 squares tall, your tile is 12", and your spacing is 1/4", you would multiply 40 x 12 1/4" and 32 x 12 1/4" to arrive at the grid size. Keep in mind that for every four tiles, you will have four 1/4" gaps, or will have to add one inch. So four tiles = 48" + 1" or 49" (4' 1").
Next you will have to scale the grid to size. I created a temporary room using interior walls. I placed the walls to form a rectangle with the inner wall measurements to the size of the grid (40' 10" x 32' 8"). Then I stretched the grid to fit into that room to make the grid of tiles to scale. Selecting the tile is a bit of work, but once you can achieve this, you now have a tile overlay that you can drag over your room. I used the arrow keys to manipulate the placement in order to find a best fit. This is especially helpful when you have different walls, doors, and rooms. By doing this, you don't come up with a surprise like having to make a 2" border around a wall or cut a 3" triangle for an angled wall.
While this might not be a perfect fit, it does bring you into the ballpark. Sadly, Home Designer doesn't have layers, and that should be a feature to include. This does beat the price of the big ticket flooring estimator software rentals though.