We use the BMFA handbook recommendations for our noise tests, but increase the distance to 12M at the rear and include a variable for the size of the turbine, one thing I have experimented with is different efflux nozzles, they can make quite a difference to the noise output, but also thrust output as well, one other way is to overpower your jet, a 120 at half throttle makes less noise than an 80 at full chat.
Mike