Small tubing benders have hash marks. When you bend tubing like I do everyday the hashes become second nature. That said it's very easy to take a die grinder with a cutoff wheel and put some hashes on your tubing bender for cages as well. All it takes is to bend a couple pieces of scrap to determine left start, right start and center, then transfer those marks to your die and mark the spots. You can buy all kinds of tools, but really all you need is a protractor and a level. When visualizing and measuring for hoop, focus on where center of tubing will be, be sure and leave yourself outside room and you will be fine. Complex bends like windshield bars look tough, but really aren't. Just use the protractor to figure the angle, roll it to that angle in die and have at it