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.