One of those niggly questions I've always wanted to ask. How do you accurately decide the mark that the conversion kick will be taken from? I know it is in parallel with the place the ball was grounded by the try scorer, but is there a set distance away.
The simple answer is 'no', there is no set distance.
For a conversion attempt the kicker may place the ball anywhere on a line that runs from the goal line, at the place the try was awarded, back down the pitch, parallel to the touchline. He can go as far back along that line as he wishes.
For a penalty kick at goal, the kicker may kick from the mark, or anywhere along a line that runs back through it. One thing to note here, is that the defence must stand 10m back from the mark, not 10m back from where the kicker elects to take the kick from.
Thanks for your question.
The Rugby Ref