What are the correct measurements for trim pieces on my entertainment center?

My entertainment center measures 26 7/8″ by 60 7/8″ on the top with two lower shelfs measuring 11 7/8″ by 60 7/8″. I’m using simple corner guards for trim. I want them to fit together at 45 degree angles. I’ve tried twice before and the pieces were too short ( I didn’t compensate for the trim’s thickness) in one or both dimensions.