I ordered a kit from Mianne last month and waiting to receive it. I took the measuring precautions Carl posted, 1 3/4" longer. My intent is to have the top board extend out over the edge of the benchwork a few inches and the sides will fit my space nicely. I use a hodge podge of cut pieces of board, so the odd measurement doesn't matter to me. I want the front to overhang about 6 inches so I am not kicking the legs all the time.
Most people I have seen have done it like Carl showed. I have to be different, I know!