Also not all rails are the same distance from the top of the saddle. And not all saddles compress the same under your weight. And the middle of the rails on one saddle bears no relation to the middle of the rails on another. Also the sweet spot for your sitz bones is in a different place on every saddle. Just putting on the new saddle with the rails st about the same place in the clamp is as good a starting point as any. But it takes some fine tuning to replace a saddle with another one of different make and/or model. I hardly think a professional fitting would be required to accomplish it. Sooner or later you should be able to know by feel and reference to your prior best fit how a saddle is placed wrong and when it is right.

Keep in mind as well that all saddles "break in" over time whether for better or worse. It is quite common for a a rider to sit lower in an old saddle than in the same model new one. At the very least the padding will compress permanently. So knowing how to recover your preferred riding position as a saddle ages is a necessary skill unless you don't mind paying for the same thing over and over again