Look at “empty passage logic” in the documentation, and I think that’s what you want!
When using a Drawstring, by default, if a passage has no text stored in it, it will follow the first link that has text that evaluates to “True”. So… if a link’s text is <<$myVariable == 0.5>> or something… that tag would be replaced with “True” or “False”. If all links are false, the final link is followed!