Nose weights and A-frame length

I'm not a novice to towing by any means but I changed the van last year to a continental van, a Knaus Sudwind.  The new van has a much longer A-frame than my previous UK built van and I struggle to get a decent nose weight.

Based on the 5-7% rule the nose weight should be in the region of 65 - 90KG for it's MTPLM (1300KG).  If I load the van appropriately I can get the nose weight up in the 70-75KG region which I always used as a benchmark on my previous van.  However, the ride in the car is a little choppy, shall we say.  

I imagine that's because the longer a-frame has greater leverage on the rear of the car, so the greater the weight on the towball the more effect it's going to have on the ride over uneven road surfaces.

The manufacturer, Knaus, has the recommended nose weight of 40KG.  This seems a bit on the light side to me and I worry that it might be unstable, especially on motorways passing, or being passed by, large vehicles.

I've never had a stability issue with any van I've towed, and I attribute that to being conscious of how the payload is distributed and by trying to get a good nose weight, erring on the side of slightly heavier (within the car's limit) is better than too light.  

However, I accept that I probably haven't got something quite right in this instance in the trade off between the longer A-frame of the van and the effect that has on the nose weight required for a stable outfit while at the same time retaining a decent ride in the car.

Could anyone offer me the benefit of their knowledge and experience please?

I don't doubt Knaus know what they're doing and 40KG might be ok but I'm also aware that Germany might have slightly different guidelines and limits on what a nose weight should be and what any given car's towball limit should be.

Thanks in advance.