I know it is not much help, but the 10 minute blocks I have seen are square and made of 10" squares. So, you would have to put borders on your photos to make them square. That being said, since you are going to put a border on them, why don't you experiment with different size starting squares with the 10 minute block and adjust until you have close to what you want and then add the borders on the pics accordingly?
I am sure that some math whiz on the board will come up soon with some exact measurements. Hope you get the help you need.