I put the measurement of a 56 cm and 58 cm frame in a calculator to try to understand how much it would change (as my saddle will be lower and therefore a but more fwd) and many other changes. In the end I got the attached result but I don't get how much the size really change. Given same saddle height and a handlebar level with saddle how much does the distance really change between the saddle and handlebar (which I assume is the really important measurement)? Given frame differs with 2 cm in seat post I calculate my saddle will be 2 cm lower on 58 cm frame (in relation to the top tube) and therefore handlebar 2cm lower (2cm less spacers needed).
Geometry of the frame is in this link:

http://3.bp.blogspot.com/-Nap-CvCrmP...metry+copy.jpg

Any help is appreciated. I should probably add it is a road gravel rando bike build.