You might try this:

Put on your kilt, put on the jacket. Measure down your back to the top of the open part of the pleats.
(Measure down from the top of your collar to the top of the open part of the pleats, i.e., the bottom of the sewn part, i.e., the fell, I believe.) That number should be between 25 and 30 inches, quite possibly 26 or so. I would ASK the tailor to chop the jacket to there. If possible, show the tailor ( via picture) how the kilt sits on you and how the jacket sits over the kilt- especially in back.

If you stand in front of something like a book case, it should be easy enough for the tailor to gauge, looking at two pictures posed the same way, one with the kilt and no jacket and one with the kilt AND the jacket, whereabouts the jacket should end. If you really want to go in for overkill, you can stand next to a series of lines one inch apart, so that the tailor can measure from the pictures. This would require large format pics and a lot of unnecessary effort. Chances are, they can gauge by the ends of your arms or your sleeves as easily as anything else.

As someone hinted, the pleats on a low rise kilt open at about the same place as on a higher-rise one. What changes is the height of the waistband and the length of the sewn part of the pleats. Thus, the jacket comes to the same place on your torso, or close...

I hope this has been helpful, or at least not confusing... I know less than MacBug.