if a Warlock is attacked for a base 1000

So, if a Warlock is attacked for a base 1000, let's presume the UR glyph reduction is applied first, removing 100 damage. Then 900 damage is incoming, 20% (i.e. 180) is split with demon. This would mean that 90 damage goes to Warlock, and 90 to the Demon. There is
720 damage left to Warlock.
www.euwowgold.fr
Total damage to Warlock seems to be 720 + 90 = 810.

Which makes it seem that the use of SL and UR glyph reduced 1000 damage by 190, or 19%.

This is much less than 30%.