Or, stick to sites don't just measure system draw at the socket but who use one of these:
Like ht4u.net, behardware.com, hardware.fr, and techpowerup.com. Of course that does mean waiting for those sites to review the cards overclocked. Never understood why some sites cannot be bothered to give real numbers. Makes their power draw results incomparable since even if one sites keeps an old system with a constant CPU & mobo, other sites are unlikely to that same system.
And no, taking system idle with iGPU vs max draw is not accurate either since there's no way of saying which part of the difference is due to the maxed out dGPU being reviewed and which part is due the CPU having to do work to max out the dGPU.
On caveat I can think of: certain combination of cards and their drivers may offload different tasks to the CPU. So for calculating which PSU to buy the max total system power draw may still be (somewhat) relevant. 'Somewhat' since draw at the wall tells you nothing about how much 12V for instance your PSU has to provide.
Oh and one final thing: measuring at the wall, the actual component draw from PSU will obviously depend on the efficiency of the PSU but the efficient differences between 110V and 240V. And the PSU's efficiency at certain % load. And... Just too many variables really.