With elevation is meant the altitude of the object (as if using an alt-az mount), this -does- change during a session.

Also, if you take refraction into account, will you then -not- take it into account when using narrow band filters? There the effect isn't measurable.

Now a question, would you consider to focus during imaging or quickly adjust the focus in between lights?
Personally, I don't use very long exposure lights, especially with Starlink up and screwing up our images.