You are describing a conditional value list, or a dwindling value list. Search this forum for those words and you will find a link from the super talented PhilModJunk for his 'Adventures in Filemaking'. There are brilliant, elegant exact examples.
This may be the correct link: https://www.dropbox.com/s/oj15fqp1uakg0d7/Adventure%201%20CVLs.fmp12?dl=0
That's the correct link. It shows 10 different ways to set up a conditional value list. What ebethram describes is set up in the file as a "basic conditional value list".