The viewport ratio in cad is 1:20, so how much should I write in the a3 box?

You printed in typesetting, which means you must be a novice. You should learn to print on the model, which is the most efficient method.

Talk about the printing scale skills in the model. Many novices will make a mistake, that is, using SC to scale the drawing object to the desired proportion, which is a very serious mistake (why is this a serious mistake, because mistakes often occur, such as you have to manually modify the drawing after zooming, and after printing, your boss says you have to modify some dimensions, and then restore1when modifying it; Modify the size of 1, scale the appropriate size, and manually modify the label. If one day you stop working in this company and hire another draftsman, people will be killed by you. If you want to reach a certain proportion, you only need to scale the outer border of the product, because the border is not a standard size and can be scaled at will (pay attention to the font).

For example, if you want to print on A4 drawings, you can draw a picture frame with a size of 297*2 10. Suppose you enlarge the frame five times, and the graphics just fit into the frame. Then the ratio is 1:5. So the scale is determined according to the needs of drawing.