Hello Tony,
You method has considerable merit, By coupling in series and then comparing the indicated loads you can establish a correlation between the two devices. If as you say you mark the special nose load gauge when the the bathroom scales show 75Kg, then you have a benchmark.
However it is important to understand that if your were then to remove the bathroom scales and the gauge now rest directly on the ground, then the indicated down force will be greater that 75Kg. For this reason you must the readjust your loading to allow for this.
For the same reason, it is important that the gauge is either adjusted to match the height the tow ball assumes when the caravan I hitched and the car is stationary and fully loaded, Or simply set the gauge to be 350mm tall (to the centre of the ball) as 350mm is lowest permitted hitch height, your car should always ride at or above this height, and so the nose load will never exceed 75Kg whilst stationary.
There is a fly in the ointment, How accurate are your bathroom scales? as a salutatory lesson, I was admitted to hospital three years ago. Immediately on admission I was weighed, and again about half a hour later on a different set of scales. The two readings were over 2.5Kg differnt. I pointed this out to the staff and asked are the scales calibrated, and yes they were, but out of date.
Knowing that the amount of some types of medication are prescribed based on patients body mass, this was worrying. I suggested they got them sorted and quickly. The second set of scales were replaced with some new ones by the end of the next day.
If this can happen to professional scales, what about your calibrated domestic ones!