Reducer for 1" fork and 1 1/8" stem
There are off-the-shelf products satisfying your needs at sub €2 price tag
:

This one is actually an adapter for stem tube but I don't see a reason that it shouldn't be used as stem quill adapter. Which means you simply manufactured this part yourself, even using the same material (the one depicted is aluminium).



Coming back to your initial question - since you've also put a length-wise cut on your adapter, the inner diameter varies within some range - it needs to be slightly larger to be able to slide it on the steerer tube. Of course, when clamped, the inner diameter is exactly what it needs to be. So in my opinion, yes, it is correct to make a reducer the way you made it.