Settings for "white cards" AKA "bounce cards"?

What are good settings for “bounce cards”, ie a photographer’s diffuse reflector?

I set a surface to pure white with ~80% roughness, but I’m not getting back as much ‘kick’ as expected.

An area lightsource?

Are you using just a diffuse shader or a diffuse/glossy setup, or are you using the Principled shader?

Nothing is purely diffuse in nature and a setup using the lower-level nodes will allow you to do things that are not at all easy to do with just a Principled Shader (such as custom mixing of diffuse and glossy components). The Principled shader itself is not a perfect rendition of all possible physical surfaces as Disney’s goal was something that could be directed by artists.

Be sure to enable the caustic effects too, otherwise you won’t get any light that is the result of glossy surfaces.

Just that, and I’m just trying for some simple bounce, nothing fancy. Just like what one would do on-set with a piece of foam-core.

10% to 15% would work better. Remember, you’re not after roughness, you’re after glossiness, so the less roughness, the better. Also, it won’t be visible in the viewport when using Eevee. Thirdly, your light source has to be pointed at the bounce card, not the object you’re lighting.