I interpret the 50g "allowance" is there to accomodate the unavoidable systematic error in the measurement of weight, to which the scales are quoted as being accurate to +/- 1%. Regardless of the quality or accuracy of the scales they will always have a tolerance, unavoidable in any measurement instrument. So if your scales have a tolerance of +/- 1% and this is defined as acceptable to the rules, then anyone turning up with a model weighing 5050g is within the weight rules.

Keith

PS. Taken from http://alpha924.server4you.de/FAI/Sp...obatics_09.pdf

5.1.2. General Characteristics of Radio Controlled Aerobatic Power Models:
Maximum overall span.............................................. ............................................. 2000mm
Maximum overall length............................................ ............................................. 2000mm
Maximum total weight, with batteries......................................... ............................ 5000g
A tolerance of 1% will be allowed for possible inconsistencies in measurement instruments for size,
weight, and voltage unless otherwise stated.