How do I filter a computed field in a report

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I have created a variable called $IncidentTypeLength and used it to create a field called "Length".

The calculation is working properly, however, I cannot figure out how to filter out values of "Length" that are < 1.  I've tried creating a filter using the same formula as was used for the variable, which is:

to_number(instr(to_char(threads.note), 'the incident occurred')-to_number('3'))-to_number(instr(to_char(threads.note), 'incident type:')+to_number('15'))

But the Filter doesn't like that the threads.note is included in the formula.

I've tried using the variable itself, but the filter doesn't like that either.