When building leaded glass panels to be placed in existing wooden frames, you need to make sure the panel is as square as the opening in the frame. The first requirement is to make sure your cartoon is squared, or has right angles at each corner.
I use a “roofing iron” as it is called in the UK. It is a steel tool about 600mm on one side and 400mm on the other. Its original use was to work out the pitch of roofs and check the same pitch was maintained all along the building. The first important thing – now that all roof trusses seem to be prefabricated – is that they still are in production. The second thing is that they all have a fixed right angle.
Using this roofing iron will ensure your corners on the cartoon are right angles. This helps in the drawing of the cartoon as you only need set the iron on the base line and draw the verticals without having to measure the width higher up the cartoon. Of course you should check that the width is still correct at the top, just in case there has been a slip.
Then you have to stick to the cartoon.
When you are setting the battens to ensure the sides are held where you want them while you continue with the leading, the roofing iron again will ensure that you have placed the battens at right angles. You choose which line is to be your base, and nail or screw it into place. Ensure it is exactly parallel to your cut line and then align one side of the iron against it. Place the other batten snugly along the length of the other leg of the iron and you know you have a right angle.
When you are completed leading, but before soldering you can check on the accuracy of the angles by using the roofing iron again at each corner to check on the “squareness” of the whole panel. If the panel is out of square, you can tap on the battens not yet nailed/screwed in place to ease it all back into “square”