From the update notes
The way that spellcasters fail after taking damage has been externalized to CONCENTR.2da. By default, any damage a spellcaster takes will cause them to fail their spellcasting.
0 Any damage
1 (1d20 + luck) vs. (spell level + damage taken)
2 (1d20 + Concentration ) vs. (15 + spell level)
Note from the Developers: This is inspired by a feature from ToBEx. Be aware that Baldur's Gate games have no Concentration skill; using that option will use a basic 1d20+luck formula for the caster's check.
I've been playing with these values, and if I'm reading it correctly it looks like with check mode 1, any damage over about 20 should be a guaranteed interruption?
from what I've noticed after trying them all, it seems far too difficult (or at least very random) to interrupt casting with anything but check mode 0...
I feel like they completely missed the point of that engine change.