Originally Posted by
The B
I don't know what 5mm comes out to by the time you get out to the shoulders, but if it's an extra inch, it's an extra inch in the right direction.
I have no idea what that's supposed to mean. What shoulders? And where do you get an inch from?
Flattening your back has to do with core strength, flexibility and whether you can comfortably roll your pelvis forward on your saddle, 2.5 mm of added saddle height won't make a lick of difference in this regard.
If you have a hard time spinning 172.5 mm cranks a 170 isn't much of a change, maybe try a 165.