When you are figuring your binding, take the width of fabric (42 you said), from that deduct at least 1 - 1 1/2 inch for cutting off the selvedge on both ends (40.5).
Then deduct 2 1/2" for what you lose when you join the strips diagonally (38). When you join diagonally you lose the width of the binding from each strip once for each strip.
Now six strips will give you 228".
Four sides at 54 gives you 216" needed.
12" is plenty of extra.
Good luck!! Ann