Changing Ingredients in Custom Recipe

I needed to modify an ingredient to correct the amount of sodium in a can of tuna. When I switched to the updated ingredient, instead of the sodium going up slightly, it added the total sodium from the can of tuna. Example: Previous level for tuna salad was 750mg. After modifying level from 160 to 250, it should have read 850mg, instead it read 1000mg. This has happened in a couple of custom ingredients I needed to modify. Any ideas?

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