For several years now I have raced my DF65 using the set-up as shown by Mike Jefferys on You Tube and the printed sheet by Phil Burgess.  These suggested that the mast rake slider adjustment should be set forward for A+ rig and moving aft for the other three rigs.   This set the mast virtually upright for the A+ rig, with a back-stay length of 951mm.

Now the top sailors, such as Craig Richards and John Tushingham are setting the slider right to the aft end of the adjustment on all four rigs, and using a back stay setting of 940mm on the A+ rig.

Why?  To what purpose?  Surely, raking the mast aft gives more weather helm, no good thing.

Don't forget that mast rake is not the only way of affecting weather helm so presumably they are using different settings for some or all of sail depth, twist, boom angles ...     You can be sure they are not sailing with an excess of weather helm although some like a little bit so the yacht is finding the lift.

You might get more replies if you put this in the DF section. 

Think of the slider at deck level as a ram to control mast bend.