While theory and calculations are nice, they all have the same flaw - Application
Practical application will help take into account environmental effects such as the target fighting back and other such attack pauses.
Example of a test would be to find a creature with a large amount of "Known" HP. This needs the assistance of the Monster Manual or others posting the HP.
One test that comes to mind is the Giant at the beginning of "The Claw" in Red Fens.
You can do this quest, only ever fight one mob and do quick resets.
Time yourself from the First Hit to when the Giant expires. Do this more than once, 10x's or more. The more times you do it the closer to +/-3% Error for your DPS (Damage Per Second). Having a Hireling to heal yourself as long as that does not attack your prey will help with a realistic fight where party support is available.
once done, take the average Seconds it took with all of your attempts and divide the Giants HP by the average seconds.
You can then also do this with master work weapons to remove the "Weapon Contribution" to your DPS.
Of course admittedly this giant is not fair to the Fire/Ice spec'd characters as it either hinders or adds extra help. So hunt around and find different High HP creatures, I recommend the 50K+ HP variety personally as you want to move it out of 1 - 3 Hit range for normal attacks, especially when trying to measure your Epic DPS.