There are some good threads discussing this in great detail but the general rule is
New Dot = (original dot/original attack) * new attack
Original means the maxed form with no emblems etc. You can also use this to work out the increase with troops. So somewhat strangely, rule of thumb is that the higher the dot : attack stat ratio (dot factor), the more dot is added with emblems.
Interestingly, that means if a hero had low attack, high dot the emblems would have more of an impact. Although there are more things to consider like direct damage etc.
As an aside, this is why I’m a bit annoyed that roc has great attack stats, middle of the road dot and annoyingly no direct damage to make use of the direct attack stat which does nothing to dot…