Dual Dimensions In A Family Table

I was looking to add this into my drawing table, but it is not working for me. Here is what i get when i try.

if "fam.inst.param.value" != "0"
$convert = "fam.inst.param.value" * 25.4

errorInvalid data type combination at right side of expression
endif

If I add in the relation as shown in this link, i get this message.

If fam_inst_param_value<>0

errorInvalid symbol 'fam_inst_param_value' found.
convert=fam_inst_param_value*25.4
Endif

Any help would be appreciated. I am using Creo 3.0.

Dennis