Don't confuse me with the facts, my mind is made up. Here is some interesting reading from the ISAF

CASE 84
Rule 18.1 Rounding and Passing Marks and Obstructions: When this Rule Applies.
Discussion of the phrase ‘about to round or pass’, room

Question:
When is a boat ‘about to round or pass’ a mark within the meaning of rule 18.1?

Answer:
The phrase ‘about to round or pass’ has never been defined precisely, nor can it be. In approaching a mark, there is no exact point at which a boat become ‘about to round or pass’ it. Almost always a boat two hull lengths from a mark is ‘about to round or pass’ it, but this is sometimes so at a greater distance too. Not only is the distance from the mark a factor, but the boat’s speed is also important, and other factors such as the conditions of wind and current an the amount of sail handling required before or during the rounding may also be relevant. Moreover the nearer the boat is to the mark the more definitely she is ‘about to round or pass’it. The answer to the question depends upon the particular circumstances of each situation.

Regards
Chet