The hem of the jacket should not go below the kilt fell -- the stitched portion of the pleats. For a person of your height, it might look best somewhere above the fell. If you are having it altered locally, I'd suggest asking the person to pin it at one length -- put it on and snap a digital photo -- pin it a little higher, snap another photo, etc. Compare the views to see which looks proportionately best on you.

For example, in this post about a converted jacket, at least one person responded that they thought the jacket was too short. This fellow is rather tall and slim, and my guess is that his pleat stitching (fell) ends about 1.5 to 2 setts below the jacket hem. Also notice how much longer his sleeves are than the jacket body. [Not knocking this conversion, just discussing relative lengths]

So the answer is "no easy answer." Look at different pinned lengths to see what best fits your body type. If your tailor is patient, you could even come back and post the photos here for advice before making your final decision.

And good going on the purchase! I also lucked into a deal on a very nice jacket for Bob, fortunately it fit right out of the bag. Take a look at it and notice the relative difference in jacket body-to-sleeve length vs. the grey one. It also covers about 1/3 of the kilt, as FM mentioned above.