Thank you all for your suggestions.
I will be visiting the links suggested soon.
I also understand Tikirocker's explanation; I will also check the 1/4 vs 1/2 clicks. Mine has ten clicks for a complete turn of the knob.
My Rawson looks exactly the same as Adrian's (only the red paint has faded).
As far as I remember the vernier trick is choosing as a reference the line with a perfect match with the scale and then read the "fractions" scale.
From the pic Adrian shared with us I read 175yds: do I got it right?
Mine is now set at 30 on the right scale and 3 on the left and it shoot straight at 300m (which is roughly 330yds): shall I assume those readings are in real yards? I am using Sellier & Bellot competition loads.