Choose the closest match: use the baking soda–salt blend if you need tang, or a pretzel seasoning if you need direct flavor.
Match the amount using the 1:1 ratio, then check the surface coverage for visual salt crunch.
If your substitute is finer than pretzel salt, pulse it briefly or add a few extra coarse grains so it stays crunchy on the crust.
Add the topping immediately before baking (or right after shaping for quick bakes) so it adheres and toasts.
Taste once baked and adjust: add a tiny pinch more salt for saltiness, or add a very small baking soda pinch next time for tang.