You need to figure the length of the borders out after you finish and press the center. Measure across the center of your top to get the measurement for the first border set (side or top - your choice). Then cut your border to that dimension and pin it in place, distributing any differences between the top and the strip evenly. After the one set is sewn on and pressed, measure across the middle in the opposite direction. Repeat for the last border. I have always done it that way and never had any problems.
Some people measure across the middle and the actual side where the border will go and average the dimension.