Fitting wood in rear bed

[yoshi]

I've measured across the back of my 49 where the wood sits & it measures 50". I'm told that the plank sizes are 4 1/4" x 2, 5" x 2, 5 1/2" x 4, 6 3/8" x 1. 9 planks in all that add up to 46 7/8" so that leaves a gap of 3 1/8". If I devide that by the 10 spaces that would be a 5/16" gap either side of the planks, would that be right?