I am confused -
You are starting out with a center block that will finish to 50 x 50 - right now it's 50.5 x 50.5???
Now where I am confused - do any of the attached illustrations look like what you want to do?
(the numbers are for FINISHED sizes)
From one of your sentences, I think it's B - but you are talking about 58 x 58 - so that would be D