Here is the angle diagram from the Flexiride page
at SouthWest Wheel:
So these are showing around 3" of travel from 'free' to 'bump' and 1.2-1.5" from 'loaded' to 'bump'. I believe it would be wise to have at least 1" and preferably 2" of clearance between the top of the tyre at its highest theoretical position and the underside of the fender - if nothing else, this should reduce how much the fender gets sand-blasted.
The Dexter advice is to provide 3" clearance from the 'full load' position. Their axle data sheets show around 1.0" of travel from 'full load' to 'shock load', so that's also about 2" clearance on the theoretical highest tyre position.