I understood that when you use a duplex charge, take the weight of nitro and triple it and delete that weight from the black powder charge to keep the pressue the same. That is, if you are loading a 450 BPE and the normal load is 120 gr of BP and you use a nitro priming load of 5 gr, then your black powder load should be 105 gr.

Now I think that is beaically correct for faster nitro powder like unique and perhaps the correction is less for slow powders like 3031.

To me, that makes sense, particulrly for weaker actions like double rifles and perhaps not as important for say a Sharps or Henry drop block.

What are your experiences?