All you need is your pant size. From there I make the saddle to your size, and add a bit of room to take into account late season clothing.
You can also measure yourself. Take your measuring tape and start about an inch or two in front of your pant seam (hip area), few inches down from your belt. Then proceed over the top of your butt to the other side of your pant seam . Think of the measuring tape as the saddle material . You want each end to be in front of the pant seam. From that number I will add a 1-2 more inches for late season clothing, so don't be too generous.
check out the video below