For most cases you don't really need to be very accurate with these things.
It is mostly about just seeing if you are in the right ballpark. You can always intentionally go over/under the spec to make sure you are inside. If you crank up the signal generator to 500% the specification yet it still works, then you can be pretty sure it will handle 100% the specification at the EMC lab even if your setup was crap and was only reaching 1/4 the power you thought it made.
Similar for EMC emission tests, you don't really need to count individual dBms. As long as you know the rough ballpark you want to be in, then you can just make sure you take care of anything that is reaching close to it. This way you make sure there is no catastrophic fail once you send it off to the real EMC lab. If you are unlucky with the test setup and do fail then you know exactly by how many dB you have to lower that offending peak.