The way I generally do it looks a bit like this:
Script...
? player distance
logic < 50: loose = true
...
TriggerRadius: 0,0
Condition: none
Type: End Loose
Condition Field: loose
That will activate the trigger as soon as
loose becomes true (which happens in the script, if the player is less than 50m from
logic)
The specific name loose isn't important, this is just a variable. You can use whatever name you like (apart from a few reserved words).
Just rememeber to define the variable (ie. put loose = false) in your init.sqs or in the init field of a unit somewhere