Unless a "calibrated " nose load gauge comes with a certificate of calibration and information regarding its limits of uncertainty from a recognised test house and a date for next calibration it is not "calibrated" in any meaningful way.
Tests carried out in the past have shown how inaccurate consumer grade compression spring nose load gauges can be, and in practice domestic bathroom weighing were also far from ideal, but were better than nose load gauges and more consistent over time.
Because compression spring nose load gauges by their very nature compress (shorten) when loaded unless by coincidence the load auge happens to match the tow hitched load height, the gause is not reporting the actual nose load applied to the tow vehicle.
For mechanical reasons the rate of change of applied nose load relative to the height of the trailers hitch, is not a linear relation ship, And the rate of change of applied nose vs height for a twin axle outfit is very much greater than for a single axle trailer.
To avoid the danger of trying to use a broom handle stood on the top of bathroom scales, use the caravan step and back issues of PC magazine to raise the scales to the required height to support the hitch. Always chock the road wheels and release the brakes.
Or use a hydraulic bottle jack on the top of the scales to raise the hitch to the correct height. (Don't forget to subtract the weight of the jack from the reading. - or some scales can be tare'd)