while the others are crazy for wanting to much, I think you are perhaps staying a little too much in the safe zone.
I would seriously recommend 4cm if you are seriously worried about proportions. 4-6cm is a good range for everyone.
under 4cm and it is hardly worth it for the cost and pain. over 6cm and you venture out past the safe zone.
and proportions wont be noticed at 4-6cm. seriously it really wont be noticed. most people wont even notice you got taller. but you are smart for wanting to lengthen both segments evenly, that is a good approach.
what is your height from ground to navel?
105cm, 3.75cm in each segment seems the absolute maximum for a normaly proportionate guy, Anything above it seems like an overkill to me. I would really hate if i went for 6'1 and sit next to a 6'0 guy with higher sitting height than me...
or have the same sitting height as a person that is 3-4 inches shorter, I wouldn't be disproportionate as Conan O'brien but in my mind ill be self conscious, Thats why proportions mean alot to me.