Looking up characteristics values in a different class

Ok..

I'm trying to grab a constant value for a material to be used in a calculation in variant config.

The following object dependency works:

$SELF.POTENCY_FACTOR_KG = STD_POTENCY_FACTOR

So if I change the 023 batch characteristic STD_POTENCY_FACTOR to 82, the 023 batch characteristic POTENCY_FACTOR_KG is updated to the same value automatically.

But I don't want the STD_POTENCY_FACTOR to be in the 023 batch class as it's not a batch characteristic. I want it as a materal specific value.  Preferrably in the 001 material class.

$SELF.POTENCY_FACTOR_KG = (001)GEN_100.STD_POTENCY_FACTOR     Class type = GEN_100,  Characteristic = STD_POTENCY_FACTOR

I also tried doing this with the 300 class type.

$SELF.POTENCY_FACTOR_KG = (300)GEN_300.STD_POTENCY_FACTOR     Class type = GEN_300,  Characteristic = STD_POTENCY_FACTOR

Is this not possible?

Craig