I don’t know if I completely understand what you are trying to do so all I can recommend is:
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the icon is chosen based on the state of your Item, not on any mappings or transformations made in the label
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if the values of the heater icons do not work for you, create copies of the icons and change the last part of their name to a value you do need. See http://docs.openhab.org/configuration/items.html#icons-dynamic
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alternatively, you can create a Design Pattern: Proxy Item and a Rule to perform the calculation so the correct icon is chosen on the proxy Item.