OpenMW 0.45 Reflect Test
(Regular programming interrupted to illustrate a point for a UESP entry, apologies for the scuffed demonstration).
https://en.uesp.net/wiki/Morrowind_talk:Reflect
Test was as follows (OpenMW 0.45, no mods installed or game fixes enabled so should be as close to vanilla as possible):
1. Make a bunch of Reflect potions with 900 Alchemy skill (each one was 46% for 131 secs).
2. Use console to spawn 10 Gedna Relvels.
3. Test once with no potions active (control test), and once with 3 potions active.
If Reflect was Multiplicative, character should have 46% + (46% of 54% = 24.84%, 70.84% total) + (46% of 29.16% = 13.41% + 70.84%) = 84.25% total Reflect. I.e. around 1 in 6 spells should still deal damage, and some should still get through.
Results:
Character died to multiple Gedna's with no potions active (as expected).
Character suffered no damage at all from both '46% x 3' trials, and killed all 10 Gedna's through reflection. Nothing got through.
Conclusion: Pretty safe to bet that Reflect is additive (46 + 46 + 46 = 'over 100%'), even if just for OpenMW, or perhaps only if potions are used.
Evidence appears to be different for Enchanted items in Vanilla Morrowind!
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