Those things have so many variables that have to be adjusted just right, plus critical fixed value components such as the frequency compensator, all of which probably drift out of calibration with time, that I wouldn't trust it for anything more than a relative power indicator.
I have a Mirage wattmeter that I calibrated using a Bird 43 several years ago, and on some ranges, the thing indicates more power output than I am running DC input! Maybe I have found a solution to the energy crisis.
I just use it to indicate relative power for tuning purposes, and for measuring SWR. The SWR indication still appears accurate when I load into my 2 kw continuous duty dummy load.
I'd just look for a good thermocouple rf ammeter. It will give a relative power indication, and you can calculate the actual power using ohm's law (assuming you can achieve a known, non-reactive, purely resistive load using the tuner).