How to measure edges and faces, correctly and precisely?

I know about the mesh display and ticking the check box before “length”. But sometimes the measurement is wrong.

Also I tried that ruler thing in the tool shelf but it’s not precise and it’s tedious to use.

How do you guyz measure edges and faces precisely? Furthermore, how can I create an edge with a specific length, e.g. 20 units long?

Thanks for the help.

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I know about the mesh display and ticking the check box before “length”. But sometimes the measurement is wrong.
Did you scale in object mode ? If so you have applied a scale factor to the object. If you scale in edit mode you do not change the object scale only the position of the vertices. If you have added a scale factor to an axis you will need to apply that scale (Ctrl+A) to have edge lengths show correctly

Ah that’s just what I needed, ctrl+a applying the scale solved my problem. Now I can model more precisely. Thanks for the help ^