Hi Will,
Measuring the nose-weight at the same angle that the caravan makes when attached to the car would be the most accurate assessment.
However, measuring on a firm, level base is probably the only option for most of us.
And depending on the type of gauge, the precision at which you will be able to measure the nose-weight will vary.
For instance, I have a small yellow plastics nose-weight gauge that fits on top of the car tow ball - and then you lower the caravan coupling onto the dummy tow ball on top of the gauge. This gauge has marks for 0, 25, 50, 75, and 100 kg. And I guess you could read the gauge with a precision of 5 kg - and from it's manufacture I wouldn't like to make any assessment of it's accuracy. So if you use that gauge, it wouldn't matter how you set up the van - they would all give the same reading.
If you are using a set of bathroom scales - then these should have a precision of 1 kg - but bathroom scales are notoriously inaccurate - often differing by several kg from machine to machine.
Accurate and Precise - jump on the same bathroom scales many times - and they are precise if they record the same weight - but if there is an offset in the balance (a bit of friction - or you jumped on the scales and damaged them or they weren't manufactured correctly) then that reading may not be an accurate representation of your weight.
So nose-weight measurement - unless you have access to accurate, precise, and recently calibrated (probably electronic) scales - then you should impose a tolerance of 5 kg on your readings.
Robert