Actually, I think this is much simpler than I initially recognized.
You are using {{ and }} in the expression builder. You should not include the curly braces. Instead, the right side of the expression should be custom.NowLess10Min (instead of "{{custom.NowLess10Min}}"). No quotes, and not curly braces.
It’s probably as simple as locking the right side and selecting the field instead of typing it free-form.