Yes always -5%. Everyone gets a -5% Physical Resistance penalty as it seens, I didn't read anything on depending on a skill, not related to this specific penalty at least ( the -5% Physical resist cap )
There are others buffs/debuffs on the Magic Reflection spell that are based on other skills, according to Uoguide, but that's something I didn't test so not sure if its working as it should.
I think that this one explains better the spell, also explain the formula used, but as I said, this part I didn't tested so idk how its working.
www.uoguide.com