Assuming you are using their foam shower pan, it's fairly easy to determine the needed height of the riser. The drain itself should fit inside of the recess in the pan.
One thing to make sure about, though, is that the floor beneath the shower pan MUST be perfectly flat and level, so your repair needs to be done carefully, and the areas around it that the pan will cover need to all be flat and level. This is one reason why most of the pros will tend to make their own pans out of deck mud and then cover it with Kerdi membrane when building this type of shower. You can level the whole area and get exactly the needed height.
There's a ring in the shower pan that can be snapped out of the middle that would allow you to position the drain at the proper position and height.
Another big issue is to ensure that the riser is perfectly plumb. That's true for any type of drain, not just a Kerdi one. The issue with any of the similar drains for this type of install is that they are quite large in diameter, so any minor error in plumb will cause the drain to be cocked. Remember that you will need room around the riser so that the hub of the drain can be pushed down and that the drain is somewhat of a funnel. Whe installing it over a wooden subfloor, they call for a 5" diameter hole, so something similar when filling in your concrete is called for. You could put a sleeve of 4" pvc centered around the riser - that would give you 4.5" hole once you pulled it back out before things set up too hard (you might be able to leave it in place, but then it would be hard to apply the cleaner and cement).
