Is this a correct assumption

This is not really helpful but it’s to do with moments about a point. The point I guess is the caravan wheel. Now it’s over 60 years ago that I studied this so I’ve forgotten the rest.

Cornersteady was a maths teacher so he might be along soon to throw some light on the subject 🤞

How about using a piece of wood that is wider than the socket 

Think "levers".  The longer the lever the less force is needed.  

So further out (the hitch) is less than further in (jockey).  So it seems possible to adjust loading to give the desired setting and measure at the coupling, swap to the jockey wheel and get a reading there. Then use the value you found for further jockey wheel measurements.

Difficulty is - what way the jockey wheel points affects the measurement, so make sure it is always pointing same way.

It could be calculated as Papgeno says (thanks btw) but with the hitch and jockey wheel being somewhere between 30cm and 50cm (?) away there won't be much difference I would imagine. I would actually if I had my caravan at home go out and do a test.

I did once buy a nose weight measurer that had a  hemispherical top that fitted into the hitch if the OP is worried about any damage.

I'm in the "will it really make that much difference" camp. I always check our noseweight before setting off using a Milenco gauge.  I aim to get it to about 90kg but if it's a few kg more, I'm not that bothered.  If it's a lot less it tells me that Ros has done it again and put far far too many clothes in her wardrobe at the back! That usually leads to another of our 'discussions'.

The car can take 140kg so the only real thing that concerns me is not exceeding the 100kg limit on the Alko hitch.

One thing I have always wondered? 

If the nose weight of the c/van is as recommended by the towbar weight limit ,it must surely  go well over on undulating/poor roads when travelling

The difference is considerable and you need to take it into account.

My hitch is 3.15m from the axle. With a reading of 65kg at the hitch these multiply out to 204.75.

Divide the 204.75 by my axle to my jockey wheel on floor position of 2.4m and you get a reading of 85.3kg.

I have an ALKO Premium jockey wheel and when I see it reading 85kg I know the load on my hitch is what I want for my lightweight van.

These jockey wheels have the advantage of also being wide and are useful if you have a mover fitted

Yes it will.  That is why one should stick to close to the stated nose weight limit as it was defined with that large variation when towing included. 

I’ve used this method⤵️

5% to 7% is the common range quoted. But that is again a very wide range. For a 1300kg van that would be 65kg to 91kg. The first you could lift and would be withing the vast majority of most cars max hitch load, the latter you could not and would be over that max hitch load. 

It is not a simple issue and needs thought and time to get correct. But once sorted you are fine.

Colin