It doesn’t make a difference. Most people that I’ve seen stop at their chest. Ultimately, what matters is the devotion behind the prayer and the gesture, not the exactness of the gesture itself.
I would say belly for both Eastern and Western traditions. In the end the act isn’t any less meaningful if you stop at your chest, but it’s not really a cross shape unless you go further down.
When I was in Egypt, I had a Christian there tell me that they did the sign of the cross by touching their head, then one wrist and then the other wrist. That way it wasn’t so noticeable.
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