I think you're mis-interpreting what I'm trying to say.

If you had enough DTR to increase damage by 10%, Ruin would go from 80 potency to 88 potency. You cast ruin, you get 80+8 potency all delivered at once.

But a DoT has a potency value that is delivered on each server 'tick'. So, let's take Bio for instance, which has 40 potency. Perhaps the DTR increase of 10% is delivered over the course of the spell, so 10% of 40 potency over 18 seconds worth of ticks. (6) So, 0.6666666667 potency added per tick, as opposed to 4 (10% of 40) added to each tick.

Might help explain why it seems to take a larger than 'normal' (the amount necessary to see a damage bump on non-DoT skill) amount of DTR to show an increase of +1 on a DoT, and would make the problem of truncating DoTs an even bigger issue.

The thing about truncating, is that it is almost impossible to tell the difference between potentially rolled over truncated damage, and the simple RNG aspect of damage.