"...but can't for the life of my figure out how to make FM use the contents of one field to look at another. Any ideas?"
You can do this by using Set Variable to capture the contents of the field before entering find mode or you can use set field to copy the field's data into a global field before you enter find mode. Either way you can then use the data to build a find request or, in your case, series of find requests.
Since you've already got a list of values separated by carriage returns, there's another option that might be even simpler.
Create a selfjoin relationship.
TableName :: PRnumberRange = TableName 2 :: PRnumber
Now you can use
Go To Related Record [Show Only Related Records; From Table: "TableName 2"; Using layout: <Current Layout>]
To pull up a found set of all records listed in PRnumberRange on the current layout. Note that if there are no matching records, the <Current Layout> option results in an empty found set.
GTRR is a very complex tool with numerous undocumented issues. The above example, should be safe. If you want to use this tool in other situtation or with other parameters, please read: The Complete Go To Related Record