When a player is damaged by ES, the resultant damage to mana is given by the following formula:
Damage to Mana = total damage * ES% * TKMod
TKMod = 2 - (6.25 * TK)
Where ES% is the percentage of damage absorbed by ES, and TK is the number of hard points invested into Telekinesis. With a slvl 20 Telekinesis (which will be assumed for all future calculations), TKMod = .75.
Anyway, the Mana *BOOST* that results from the DGtM mod is given by the following formula:
Boost to Mana = total damage * (1-ES%) * DGtM%
Where ES% is again the percentage of damage absorbed by ES, and DGtM% is the percentage of Damage Goes to Mana from your gear.
Now, the goal of this excercise is to find a value of DGtM% that will entirely offset the mana reduction from a certain level of ES. As a result, let's set Boost to Mana and Damage to Mana equal to each other and solve for ES%.
Total Damage * ES% * TKMod = Total Damage * (1-ES%) * DGtM%
ES% * TKMod = DGtM% - (ES% * DGtM%) _______ (dividing both sides by Total Damage)
ES% * TKMod + (ES% * DGtM%) = DGtM% _______ subtracting (ES% * DGtM%) from both sides
ES% (TKMod + DGtM%) = DGtM% _______ factor out the ES%
ES% = DGtM% / (DGtM% + TKMod) _______ divide both sides by (DGtM% + TKMod)
Assuming a TKMod of .75 (20 hard points in Telekinesis), we get the following data points where DGtM perfectly offsets Energy Shield.
50% ES (slvl 7) is offset by 75% DGtM%.
75% ES (slvl 20) is offset by 225% DGtM%.
80% ES (slvl 25) is offset by 300% DGtM%.
82% ES (slvl 27) is offset by 342% DGtM% (this is the maximum achievable offset).
Playtesting confirms that with the proper amount of DGtM, a sorceress's mana ball won't budge, and as long as she has enough mana to absorb the initial hit, her ES will remain active. I have yet to test the DGtM setup against a mana burner, but I'm very interested to see what happens. If the DGtM triggers AFTER the mana burn, it's possible that a decent amount of DGtM will make a Sorceress mana-burn proof (although I suspect it's more likely that the DGtM will apply BEFORE the mana burn).
Now, where can a Sorceress *get* DGtM gear? Actually, a lot of it is pretty cheap. Here's a list of the best items at each slot.
~Nightsmoke Belt (50% DGtM)
~Naj's Armor (socketed with Ith) or a 4os Armor with 4 Iths (60% DGtM)
~Howltusk or Cow King's Helm socketed with Ith (50% DGtM). A triple-Ith helm would have 45%.
~6os One-handed weapon stuffed with perfect Dun jewels (72%). The Rift runeword provides 38% and has the advantage of not being unable to kill anything.
~4os shield stuffed with Iths (60% DGtM)
~Angelic Ammy (20% DGtM)
~Carrion Wind (10% DGtM)
~Aldur's Advance Boots (10% DGtM).
The Problems: DgtM only affects physical damage, so elemental attackers will still tear through your blue orb in a hurry. DgtM only affects the damage that you receive, so PDR and %DR will make DgtM pretty useless in a real hurry.
Final thoughts: While it's pretty much useless with a very high level of ES, I've been pretty happy with the results of pairing DGtM with a low level ES, like the kind you get when you don't put any hard points in ES, but you keep a Memory staff on switch.
tl;dr- it works just fine, there are just extremely harsh diminishing returns on DGtM with a high-level ES.