The method you have described will not  give you  an "accurate" reading.   There are three  significant  deviations from the official method of determining the accurate nose load:
1 Whilst  bathroom scales will probably be both more precise and accurate than  most compression spring so called nose load gauges,  they can only  be classified as Accurate if they have been formally calibrated. Admittedly in the grand scheme of things probably not really necessary.
2. The official methods of determining  nose load do call  for the process to be carried out  on  level ground,  they  do not  specify the caravan has to be level. In fact  it  probably  wont be level when its hitched to the tow vehicle,  The angle of the caravan to the horizontal does affect the applied nose load.
3 It  is important  to match the hitch height  to its real towing  height,  so having  some means of adjusting the height  of the gauge is important.  If using  bathroom scales,  the scales should be raised up so the measurement  device is supporting the hitch. If you use magazines to set the level,  put them under the scales not  on top.