The flat portion where the carriage bolts to the bottom of the saddle is a factory surface. That is where you check to make sure the top of saddle flat ways are parallel or coplanar . You have to be a detective and turn the saddle up-side down and look for the worn and unworn areas.
Have you ever heard of the South Bend Ridge on the bedways? That is created by a low area on the saddle where it is clearance. That would be on the bottom of the V way (upside down remember) now look at the top of that V and you will see a unworn spot that you can depth mike that ridge to the worn area. You need to make a sketch and write down the numbers so you can figure out how much material is worn off the saddle.
One thing you have going for yourself when your rebuilding, you can follow the builders original scraping and geometry.
I scrape machine for the most part from bottom to top. So we are assuming you already scraped or had the bed ground and you are now match fitting the saddle to the bed. On a worn machine like a SD usually is I apply a wear strip on the bottom of the saddle to raise it back to the original center line. You had the bed ground or scraped and you know how much was removed to clean up the deepest wear.
On your sketch you have that written down. Then you measured the wear in the from the original unworn areas. Add those up.
Then apply plastic shim stocks or small shims of turcite under the worn areas, and slide the saddle back and forth indicating the surface we originally talked about and add more shim until it indicated parallel to the bed.
Under there is a keyway you can also use to measure to indicate to keep it moving in the same as the original travel . After we do the bottom we scrape the top parallel to the bed (we leave the headstock ends lightly higher) and coplanar to each other, then scrape the dovetails and use a square on the lathe bed or setting a blade square bottom against 2 dowel pins and then we push the saddle back and forth indication the blade of the square. we scrape the front dove so it is not perfectly square. Scrape it so when you face a part it cuts the part concave.
A couple of You Tube shows that were taken during a class shows this. I will add them in a bit.
