You say that your center is 51.5 x 60 finished. You want the first border (finished) to make the top 74x78. That means that the first side borders need to be 11.25" finished (11.25x2+51.5=74) and the top and bottom borders need to be 9" finished (9x2+60=78). Did you want your top and bottom borders to be different sizes?
Regardless, suppose you want to have squares on point along a 60" side. The diagonal measurement of the square is the width of that portion of the border. You want the border to be 11.25" finished, which means that the squares need to have a 11.25" diagonal. But an even number of these squares will NOT add up to 60". If you use 6 squares with a 10" diagonal, they will add up to 60", but that will not be the width that you want (it will be 10" wide). Do you see the issue? You can't always start with a set center and put squares on point around it and have it be the width you want, all at the same time.
What you can do is change the width of your center piece by adding a little or subtracting a little. I hope this helps.