Originally Posted by
fa63
Seat tube angle should be a factor, provided that you are using a non-zero setback seatpost, because saddle height is most commonly measured from the BB as a straight line along the seat tube and not as a direct straight line from the BB to the saddle. I edited my original post to reflect this. Also, if all you want to know is the difference between the old and new saddle height, knowing the old saddle height is not necessary (i.e., you can re-arrange the equation in terms of z-H).
I respectfully disagree. You measure saddle height from the BB to whereever your butt hits the saddle. Any other method is wrong, because setback is a factor, even on a non-setback post.
As for the equasion, you're dealing with an arc. You want the saddle position to move in an arc in order to retain the same distance from the BB, but sliding it forward in the post moves it linearly, so you have to correct for the variance. The reason the original seat height plays a factor is because the linear movement makes a different outcome in arc degrees depending on how long it is. Seat tube angle is not needed because the distance moved will determine the angle CHANGE, regardless of starting angle.
In reality, it's easier to just measure the seat height before you move it, then adjust it to the same height after the move. I promise you, if the OP does it, there will be less than 1mm to move up.