Formal is generally defined as Tuxedo. "White tie" and "black tie" are levels of formal.
Semi formal is generally referring to functions requiring a dinner jacket.
When kilted, if you do not own, or wish to wear, highland dress, a dark suit coat will suffice. Black or Navy is preferred.
White or pale cream shirt, with bow tie if you have one. If not a solid tie or one with a small pattern.
Cream hose, black wide belt and black dress shoes. The shiner the better.
Semi-formal is one step up from Business dress. You should look better and dressier than a politician on TV. Think of Sean Connery as James Bond when he goes to play Chemin-de-fer in the movies.
Remember this Semi-Formal event is classy. Probably with speeches and ballroom dancing afterwards. It is a time to dress to-the-nines. A Maple Leaf Tartan Kilt is perfect due to the muted colors. Be sure to wear the dressiest sporran you have. Belt, shoes and Sporran must match