When I build a frame I make a drawing on a large sheet of paper. I first use a ruler and compass to draw out the frame’s C to C measurements. Then I simply use a ruler and straight edge to draw in the tubing widths. It’s a painstaking process but isn’t all that bad in the grand scheme of a frame build.
I use the dimensions from this drawing to determine where I need to place the miter templates.
Tips:
Obviously the drawing needs needs to be quite accurate. Spend time on it.
I bought a large compass specifically for making this drawing off of some math oriented website. It was only $10-15. Small compasses like the ones you used in high school aren’t nearly accurate enough in my opinion. They are just too small with too closely spaced degree markings to work well for a full-scale bike frame drawing.
For free paper that works great go to your local Home Depot. They have a large roll of paper you can use to cover the inside of your car if you bought something that can get your car dirty. It’s on the same cart that you get that plastic cord to tie large items to the top of your car. Just tear a few feet of that off, it works great. Obviously buy something while you are there or else it’s the same as stealing.