Yes I see what you mean no connection to expression options.
Suggest creating a formula in Expression Editor to work out the end limit from a number of turns expression and us the Limit method.
For some reason the expression is not accessible from the helix dialog.
Probably worth a PR to GTAC
The expression is created and editable from the expression dialog.
If you copy and paste the expression and later on change the expression it will have no impact.
(it will not change the turns in the helix)
Unless the original question was regarding linking the Turns value to an existing expression.
There is no need to create an additional expression, the helix feature command does create an expression. The only difference is the expression editor "=" shortcut on the "Turns" value is not available from the helix feature dialog. (oversight by programmer?)
In my screenshot of the expression editor there is an expression "p13" that was created when the 'helix feature was.
Open the expression editor and edit the value or rename it there.