I also recognized sling placement as a concern some years ago and decided to take a hint from the big boys, big sportfish etc, and screwed on two small plates saying "Sling" after a particularly well executed haul out with proper sling placement. I recall that the fwd sling was shortened as it goes just fwd of the keel while the aft sling is placed just fwd of the bottom rudder fitting and needs to be regular length.
This unevenness of the slings allows the boat to be raised level whereas with both slings the same length, the fwd sling goes up a long way to keep the boat level.
I will say though, some Yards are kind of set in their ways and will do it their way anyway regardless, so good luck with the impending haul-out.
I don't own "Flicka" anymore, so I can't send any pictures as to where exactly the plates are installed, so again good luck.
Hans Heiduck