This calculator will help you understand seatpost setback needs, seatpost exposure, and total saddle rail height.
For the purposes of this entire site, any reference to "Setback" or "Saddle X" does not equal the typical "saddle nose to BB" measurement value.
However, if you know your Saddle Nose to BB measurement, take that number and add it to the horizontal distance between saddle nose and Saddle Rail Center (SRC) to find Saddle X. To find Saddle Y, measure SRC to the floor, then BB center to the floor and subtract those two values.
Setback vs STA: The horizontal distance between the Saddle Rail Center (SRC) and the center of the clamp if you were using a straight / zero offset seatpost. A negative value indicates the SRC is further back than the clamp, and a positive value indicates the SRC is further forward than the clamp. As an example, if the value is -30mm, you'll more than likely require a 15-20mm setback seatpost to achieve that position.
Effective STA (eSTA): The actual seat tube angle created by drawing a line from the bottom bracket to SRC. This combined with "Setback vs STA" can be useful if building a custom bike when deciding if you want a straight or setback seatpost.
BB to Rail: The total distance from the bottom bracket to your saddle rails, measured along the seat tube angle. Useful for bikes with Integrated Seat Masts that may require cutting the seat mast, or for determining which seat mast/post length is needed (common situation with Trek bikes)
BB to Saddle Rail Center (SRC): The direct distance from the bottom bracket to the saddle rail center coordinates (Saddle X, Saddle Y). This is calculated using the Pythagorean theorem and represents the straight-line distance from BB to SRC position.
Exposed Seatpost: The length of seatpost that extends above the frame's seat tube. This is calculated as BB to Rail minus the frame's specified seat tube length. This has implications on seatpost compliance/flex for comfort as well as determining how long of a seatpost may be needed.