How far down does your standard back and breast go before srticulated plate?

It seems it would not go down too far past the solar plexus to me to allow flexibility, but I really have no idea.

My breast plate ends about 1-2" above the navel, it is a late 14th cent design.

Breastplates end somewhere to or just above the naval. Basically you need to be able to bend so the naval is a good point for it to end. You do not want to make it too high as it takes away from the protection it offers you with no real advantage in mobility. Backplates usually are made to match the breastplate. I usually take the measurement from my collar bone to my naval for breastplates I make.

RPM