actually there is an accurate ruler . here :
[REL] Improved Malloy's Ruler
@Archer9 : you will find the instructions of how to use it inside its .rar
you can use it at any scale of navmap but the most comfort scale is the 0-500m of map . study carefully the readme and you will figure out that the tool can be used at any scale with the proper attention at the divisions of map's scale
@Fader_Berg : you are drawing lines on paper and not at sea !
if i give you a ruler and a paper and ask you to draw a 16,85 cm long line ...you can't do it with extreme accuracy ? it is the same thing ! Navigator officer has a ruler and a paper and wants to draw a 20cm long line....why not to be able to it with extreme accuracy ?