An easy method of mapping where stuff on an already carved face sits in relation to an uncarved one.

Take your carved mould piece, and put some carving dust on it. (Or Talc, if you've cleaned everything away..)

use your fingers or a flat edge of something to fill the cavity and grade it off.

Put the two halves of the mould together, turn it over so the carved side is uppermost, give it a tap. set it down and take the carved side off.

(The dust is on the left, the Carved half on the right.)

Voila! you know where things need to be!

and you get nice matching halves, 

NB if carving an offset shoulder entry sprue as shown here, carve your pin first, then the sprue. 
It makes a difference. 

Method II

Cut the sprue on one half of the mould.

Map that onto the un cut half

Once you have your Outline,

MARK THE CENTRELINE! And work out from there.

To map the Pin, you dont need a fully cast pin.
Just clip the sprues and shoulder in line with the centreline of the pin 

then use the clipped studs as your guide to map the line onto the uncut half.

And you are golden