I recently got a great price on a vintage(ish) Agyle jacket in black barathea, which I plan to use as the equivalent of a dinner jacket for events where a Prince Charlie and waistcoat are a bit too much.

What is considered the proper evening shirt for that mode of highland formalwear?

When dressing down my tuxedo by wearing a cummerbund in lieu of a waistcoat or wearing a dinner jacket, I usually wear a pleat-front evening shirt with a fall collar, because that is, more-or-less, the etiquette/custom/convention with Saxon formalwear.

Does that etiquette/custom/convention hold true for highland formalwear?