It looks like you did a good job with the math. I don't believe the one quarter inch difference will be noticable enough for you to worry about. If you're really worried about it, you could make up a 3 x 3 sample with scraps to see if it bothers you. That wouldn't take too long, and might put you mind at ease.
And, the bottom line is, it's your quilt, and you're the one who'll be living with it and loving it every day. Enjoy it!
Can't wait to see your finished product.