Unit ( Metric) Problem : Not able to get full 20cm im looking for days now ... 

I changed my Scene to be Metric, with Degrees and Scale to 0.001.

Objekt Display / Lines: 50 / Scale: 0.01

I need an Icosahedron with 20cm long Edges.
I get 20.019cm or 19.978cm etc and its not like i didnt try to type it in...but i guess, when I type it, Blender will use its own Units and force them into metric messurements.

I want to  have an Scale/Unit system that works on 20.xcm max characters but exactly.

Either having an easy manual scale or to have a perfect "type it in" choice would be greate.

Question aside from that, i didnt find anything regarding ( maybe because i dont know how its called) but i want to work on an objekt lets say a cube, and i want to extrude every face of it and scale it, but i want the faces to have all the same values/ same modification as different faces: im asking because when selecting all the faces and extruding it acts weird or it will just be dublicated.

x3 tanks in advance for any help
If I understand it correctly here we go:

First issue.

Blender doesn't have so precise modeling system as CAD software. So if you want correct edge lenght you will need to do this manually.

While scaling object in Edit Mode with selected edge lenght you can hold Shift to get more precision. Zooming in and holding cursor (before hitting S) away from object center could help a bit.



Second.

In Edit Mode change Pivot Center to Individual Origins then in tools choose Extrude Individual, Right Click and S to scale down each face.