Current in game tooltip DPS for DoT is: base damage * [total increased] * [totalmore] * (1 + increased effect) * (1 + increased duration / base duration) * (1 + penetration) * chance to apply, where base damage is total damage for the whole duration, e.g. 35 Fire for Ignite.
And this bit (1 + increased duration / base duration) is buggy, it was reported and should be fixed in future updates.
As for the ramp up time - yeah, it’s difficult to account for. In general ramp up time = duration * (1 + increased duration) until somewhat constant number of stacks on target, but of course fights can be much shorter.