If splicing to a thimble, size the eye to it now. Your thumb indicates how far the crossover will bury, and if you pry the core out at C, there is no need to make a mark. On the right is a tricky labor saver. Keep the point of the awl through the loop of the core as it is extracted, and measure down eight x Dia. When you are ready to make the crossover, it will be easy to find.
Old rope requires a few extra tricks.
The distance to the safety pin is only an educated guess of the distance needed to shorten the core the proper distance before the eye is laid. Sometimes when splicing used rope, it is easier to move the core the full length of the rope being spliced. You can expand the jacket and bury the crossover much easier.