(Assuming you're drawing on your design on the outside, then cutting liquid mask on the inside....)
I mark reference 'dots' on either side that I'm happy are symmetrical, and then join the dots freehand with a sharpie pen.
Where the two side meet at the front of the shell, I often fold a piece of paper in two, and cut it in the shape of one half, then open it up and you have a perfectly matched shape. I then hold this onto the shell and draw on the line before tracing it inside with the scalpel.
My latest trick, which I love, is to lay 3mm green masking tape (available from everythingairbrush.co.uk) on the outside of the shell to make a cutting guideline for the eye. It's bright green so easy to see from the inside. It has massively improved my cutting standards, and helps ensure trim lines are parallel.
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