I don’t know how I got recommended this video but if you require 4.5 blocks on one side, in order to make it square, you will require 4.5 blocks on the other. Not 5. You could also measure the hole and divide by the length of a block. In this case you would round down the number to the nearest half.
From my experience, it is best to design to use whole blocks when you have 8 x 8 x 16 blocks, to avoid having to buy or make half blocks. I found the hard way that this does not work for 6 x 8 x 16.
To use whole blocks I would either need to extend the length 8 inches or reduce it 8 inches. Since it is only 8 half blocks for the total project, I hope that isn't really an issue.
