According to my research.
Rift's damage depends on both attacker's and victim's will.
On classic difficulty ethereal has 145 will.
Damage dealt by ethereal's rift seems to be like this:
Rift damage = 6 + (145-X)/10(rounded down) (+2 against humans) (-3 against mind shield) (-2 against psi armor),
where 'X' is victim's will (including buffs).
Rift damage can't get lower than 4.
Thus, ethereal's rift deals:
6 damage to ethereal,
20 damage to robotic units (0 will),
18 damage to elite muton (20 will),
9 damage to sectoid commander (115 will),
5 damage to my support (114 base will + mind shield),
5 damage to my squaddie (174 will because of "lead by example" effect),
7 damage to my MEC (110+20 will).
(It`s all tested)
You may buff/debuff the ethereal to get stronger/weaker rift.
If you use psi inspiration on ethereal, the rift deals 23 damage to mechtoid instead of 20.
If you use mindfray 3 times, than rift deals 11 damage to elite muton instead of 18.
But my formula isn`t accurate:
rookie (40 will + mind shield) should get 12 damage but he gets 11,
squaddie (50 will) should get 17 damage but he gets 16,
sniper (111 will) should get 11 damage but he gets 10
So you can calculate damage approximately.
Just use your soldier`s will instead of '145', it works the same way.