Not a tailor or particular expert in highland wear by any standards. I normally wear a UK 52 Long or X-long so my info may not be valid for your case (my sleeves are 27" typically).

But, in all my pictures where I am wearing various Argyll jackets with the kilt the jacket sleeve seems to end at the same level as the side of the jacket body, or maybe slightly lower than the jacket body by less than an inch. Since the Argyll jacket has a pretty flat "hem" except for the front cutaway, I think you could probably use this as a rough guideline. Along the sides and in the back my jackets tend to cover about 1/3 of the overall length of the kilt when both are worn together.

Hope this helps.

jeff