I think it is a bit of an old wives tale that the aiming circle is just that and you can judge your approach angle from the ovoid or otherwise shape of the circle. At night you can't see it, you look for the blackest hole and the odds are you will find the deck there, when you get there you find the circle is partially obscured with bird poop, by this time you have almost hit the thing so as an approach aid it is next to useless.
In practicle terms the circle is sized to 50% of the deck size i.e. 16 metre deck, D value of 16. The inner measurement of the circle is 8m. If you land with the nose of the helicopter on the circle (bum line for the S61 drivers, because that's where you put it) you can be sure that the stickyout bits at the back and front will always be clear of obstruction. This only works if you land your helicopter that has a D value of 16 or less, stick your Puma/Tiger or S61 there you have more than one problem. This should't happen because we all know our particular helicopters' D value and the meaning of all the other deck markings.
Helidecks that meet all the requirements are very few indeed and teefor gage is right when he says pilots are to blame. Too many go for the easy life and don't make reports, it is so easy to put restrictions on helidecks that are poor so that sooner or later they bite and repairs/cleaning is carried out.
I can see I'm boring you so I will wind my neck in, until the next time that is.