Don't use Kohler! They are probably the hardest to install and have look good as you've found out, they require very precise location. If you don't get it exact, the part will either be recessed, stick out, or not seal. Way too much hassle for what it's worth, IMHO. Kohler also changes designs frequently, and getting parts down the road can be a problem. IMHO, they are infected with the 'not invented here' syndrom, and choose to do things their own way, regardless of the consequences. Thus, it is rare that a part stays in 'style' and therefore stock for very long. Their parts catalog looks like a small town phonebook. As a result, few dealers stock parts, so you are at the mercy of the factory, if they still have it or will make it for you.
If this backs up to an interior wall, it might be easiest to just tear it open and anchor the blocking and the el exactly where you need it after the tile is up. Otherwise, it is going to be a guess, and the consequences of not getting it right will likely bug you forever. You can set the tile around that opening first to set the plane, then fill in the others, but you can't just make the thinset layer thicker on one tile, or it will stick out, or be recessed to give you the projection you need...a major pain.