After you have drawn out and altered your pattern you will usually have to true the dart intake so that it lines up with the corresponding edge properly.

You can just guess at it, and Fall short or cut it off if too long, or you can use one of these methods to do it.

The first method is to work out the geometry with a ruler:

The second method you don’t need anything to measure with, just a reference point that is shorter than the dart leg itself and a straight edge.