There was a huge discussion a few weeks ago surrounding melee hits using the damage value before the damage was halved. We all very well know that damage is halved in the combat damage formula for all melee strikes for those that follow the topic. This after all was "just a theory" and never was proven.
Recently there has been a major discovery in the demo code involving damage being cut globally in half. This means when damage occurs in game that the server will finalize that damage value by dividing it by 2 at the end result. This leaves an open posibility that the previous theory for spell interruption for melee hits may very well be the case globally. It is highly possible that all damage was calculated in the spell interruption formula before the value was cut in half through the global method.
Here is the patch note in 1997 indicating that ALL damage was cut in half globally that is also present in the demo.
I think this is the missing link or key that surrounds the spell interruption formula...1997 - http://wiki.uosecondage.com/index.php?title=Ultima_Online_Patch_Notes_for_12/09/1997 wrote: All damage has been mathematically halved to slow down combat across the board. This takes place after damage absorption by armor.
Edit:
Batlin just informed me that he will be doing some extensive testing with the spell interruption script to get precise values being casted into the spell interruption method. This will tell us if this theory is very well true or not soon as he finishes this project.