New answers tagged stealth
Passive is "10 + all modifiers that normally apply to the check" so your original calculation is correct. Also "If the character has advantage on the check, add 5. For disadvantage, subtract 5"
You could use Miniaturization, so that your belt buckle projects some sort of force field. The Proteus mod is great for things that transform into other things, which to me always felt like a vehicle kind of property. In a way it's realistic: armor looks bulky, or makes noise, or sticks out.
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