create grid of frames with set gap size within preset dimensions

I am trying to simplify the process of creating layouts for digital photo album creation. I want a grid of frames with .125 gaps between all of the frames, however, I want the entire grid to remain within preset boundaries. So I make a rectangular frame within the preset boundary (let's say 8x5) with the frame tool, then while still holding the mouse down, I click the right arrow key twice and the up arrow key twice, giving me a beautiful 3x3 grid of equal-sized frames. Now I go to the align menu and select each row and each column separately to insert .125 gap between each picture (incidentally, is there a quicker way to do this, like select the whole grid? doesn't seem to work for me). The problem with this approach is that the grid is now 8.5" wide and 5.5" high instead of the preset size of 8x5 that I need. I know I could make my original frame that much smaller to account for this "growth", but I want to know if there is a more elegant way to do this...somehow constraining to the original size? (All frames within the grid must be exactly the same dimensions). Any help from seasoned inDesigners much appreciated!