History lesson:
Soulburn from its introduction to its absolute decimation, which happened I want to say with launch of Vetrovia, oscillated between being [10-15]% of a summoner's parse that played their abilities with optimized prioritization. Soulburn was certainly on do-not-ever-cast list last expansion and this expansion.
Its damage-gutting coincided with the change to pet HP no longer being tied to owner HP, thus pet HP going way down, but the damage from Soulburn plummeted way more than the ratio of the pet HP drop. For easier understanding, let's say pet HP went down 10x with the Vetrovia change. Soulburn damage went down by way more than 10x (we're talking over 100x), so it was not only the pet HP change that can account for the Soulburn nerf. While I'm sure the reversion of the pet HP is not in the cards for "reasons", it needed to be fixated here that were pet HP change just simply reverted, it would still leave Soulburn emaciated.
Today 1 cast of Soulburn does less than 1 tick of Vampire Bats on Conjuror (important because VampireBats ticks do lot more dmg on Necro)
Proposed change:
Damage of Soulburn is soooo far off the mark of where it optimally should be to return it to the originally-intended place on the parse - that I can't even propose a reasonable X% multiplier to it. I would recommend simply getting rid of the gimmick that it's tied to pet HP in any way since that connection has completely stopped making sense, and just set the base damage of new Soulburn equal to about 4 * [Wildfire III (Ancient)]. If used on cooldown over a period of a 2m+ fight, this will produce roughly 8-10% of the total parse like the original Soulburn roughly.