I have done a couple on graph paper. One because I was being paid for it. Usually though I have kind of an idea what I want and make up my mind as I go.
Fortunately for me maths has never been hard. I use to draft clothing patterns years ago. I can look at a quilt and generally work out the pattern. I may change a few things. I do a lot of picture quilts and I Spy quilts for kids.
I like to make something outside my comfort zone every now and then. Just wish I had more hours in the day. Would get more sewing done if I stayed off the computer. When I leave it turned off I do get some sewing done.