It took me awhile to figure out how to determine the size of fabrics and zipper I would need for the pouch size I wanted. I'm not really a numbers person so I had to draw and re-draw, measure and re-measure until I was sure. And then I made one first out of scrap fabric just to be doubly sure my calculations were correct.
It's happened to me before that I've figured sizes and then measured fabric over and over again to be on the safe side and for some reason after it's been cut and sewn, it's off and I sit there not having a clue where I went wrong. So, whenever I'm measuring and calculating, I'm always a little nervous. However, in other areas of design I can just "eyeball" things and they are right on. One example was when my husband and I were wallpapering the laundry room - correction, I was wallpapering the laundry room and he, being a general contractor, saw my methods and decided he needed to step in to make sure things turned out right. Well, I started putting up a piece of wall paper on the wall and he said, "Wait! Did you measure and mark the wall to make sure the strip goes up straight?" I said, "No, I know it's straight by looking at it." Well, we debated for a couple minutes and he decided to pull out his tape measure and level and sure enough, it was right on. I decided not to rub it in and he decided that I had just gotten lucky. I knew better. ;)
So, being more visual than anything, the most challenging part of sewing for me is when I'm not working with a pre-made pattern and I have to start adding measurements and seam allowances and 8ths and 4ths and halves together. It's slightly embarrassing but here's my worksheet.
As you can see, I had to draw it over and over again, mainly so I could visualize it, and keep redoing the numbers. Sheesh, my 12 yr. old son could have done this in his head. He and his brother take after their dad that way.