what T-mar said. I ride long cranks and big frames. I need a zero offset post to position my saddle angle correctly. I don't think I have a frame with a seat tube steeper than 73 degrees. On my road bike the saddle is clamped rear of center on a zero offset post! it looks ugly, so I built my first bike for me with a 74 degree seat tube so I could clamp my saddle in a more aesthetically pleasing position. Once the saddle position is dialed in, it is time to dial in reach and bar height.

Like you have noticed you can change the effective seat tube angle by sliding the saddle fore or aft. This is the reason bike fitters have been championing stack and reach for years and most manufactures of road bikes now report stack and reach in the geometry. The reason is that if you report the position of the center of the top of the head tube from the bottom bracket you can actually compare one frame to another for fit without working out how frame angles affect the fit. If you see a professional fitter they will dial in your saddle position first and then your handlebars.

Maybe adjust reach by moving the saddle. This is just wrong. However, some people are so insensitive to position I am jealous. My body is plauged with old injuries and asymmetries and it very sensitive to fit over long distances.