I’ll chime in here as I keep colonies in single langstroth deeps and super with shallows. I also under super and take them off from the top as they are ready. 
> 
> I wonder how much honey by weight is in a shallow super - I use mediums.

Three 9 frame supers is 60-70 pounds for me. Generally three supers will slightly overfill a 5 gallon bucket. 

> And I wonder to what extent the honey is capped when you pull it.

For me, this depends on the flow and the humidity and the temperatures, but mine are typically 80% capped. I test with a refractometer. Generally 16% or less moisture during the summer. 

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