I don't know how racedoc figures 25g as being ideal, but if it is, then there is plenty of leeway up or down from that number where the resonant frequency would not be appreciably affected. The math tells us this.
At the risk of being pedantic, which this is, the effective mass of the moving parts is equal to the net effective mass of the tonearm, headshell, screws, and cartridge, plus the effect of the counter-weight on the net effective mass, taken together. The math for that is tricky (I know that I don't know how to solve it, anyway), so most of the time we are just approximating a value based on the given effective mass of the tonearm, if the manufacturer is kind enough to give us that piece of information.