Your close...you add up the four sides of fabric and that gives you your total length of binding...add a few more inches for all your connecting points.
Do you do your binding on the bias or using WOF? I use WOF...so once I have my total length I divide it by how many inches in a metre of fabric...so about 40 inches. That tells me how many WOF lengths I need...then I just determine how wide my binding is to be. I like a 2 1/4 inch binding, folded. So to be sure I have enough fabric to square out the cut edges I round up to 2.5 inches.
As an example, if I've determined I need 5 WOFs...I multiply that by 2.5. Which would be 12.5 inches of fabric length I would need.
If I've determined I need 10 WOFs...x 2.5 = 25 inches of length of fabric.
Does that make sense?