I think what you are doing will "put you in the ballpark". And after all, it is all that you can do with that meter.
I also don't think that they expect you to nail the 50WPEP with a calibrated instrument, either!
You could also measure DC Voltage and Amperage at the power supply and multiply by efficiency factor, or even get into that ballpark with the AC mains voltage/current, but again, I think that you are giving it the old college try here.
But I don't know what the situation is in your country, if you actually have inspectors or the like who will knock on doors demanding to use their equipment to measure your power output, that would be another story entirely.
Outside, they aren't going to be able to tell all that accurately whether your signal is 40, 50 or 75W PEP at the radio antenna jack.
KE3WD