What you describe attempting is called a conditional value list where the value in one field limits what values appear in the value list specified as the format for another field. While there are a number of different ways that you can set up a conditional value list, the basic, relationship based conditional value list would need a second relationship linking Calc to a different Tutorial: What are Table Occurrences? of Equation so that this new occurrence of Equation can be used as the source of values for your value list.
Calc::adultThreshold = Equation|Threshold::AdultThreshold
You can then set up a "Use values from field" value list listing values from Equation|Threshold with "include only related value starting from Calc" specified.
To learn more about different options for conditional value lists, see: "Adventures in FileMaking #1 - Conditional Value Lists" This file demonstrates 10 different ways to set up a conditional value list.
What I was trying to understand was why instead of having a TO relating the IDs for the popup field and a TO relating the 'Adult Threshold' for the value list using only values starting from Calc (which works), why it doesn't work to have both relationships to a single TO (the popup won't populate).
Because that defines a different relationship. Double click the relationship line and note what you see for the match fields:
You get something like this:
Table1::Field1 = Table2::FieldA
AND Table1::Field2 = Table2::FieldB
You only get records matching if the values of both fields in Table 1 match to a record where both fields in Table 2 match them--hence the "and" operator. That won't work for either of the two relationships you describe in this thread.