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That brought it home for me, nice explanation there.
I started up a new drawing to create an example in and wouldn't you know the ISOLATE command worked there just fine. I don't know why I keep having trouble with populated files, I need to get in the habit of trying my issues in a new file first before posting. Thanks for the assist.
You have to make sure that the final units of your formula match the unit type for the parameter.
For example, notice that the unit type of JD's "Counrt" parameter is "ul", and the final units of the formula "Length / 1mm" is also "ul", so it works. This formula will not work in a parameter that has any other unit type.