How do I adjust for overexposure using physically based lighting? I am trying to get my head around physically based lighting in Blender.

I read somewhere the Cycles Sun Lamp is measured in W/m² so
assuming that bright sunlight is measured at 120000 lux (https://en.wikipedia.org/wiki/Daylight) my intensitiy for the blender sun will be (given that 1lux = 0.0079 W/m2 for sunlight) around 980 W/m². This matches more or less with the readings from wikipedia, that states that the max sun intensity depending on its distance from the earth is 1050 W/m² (https://en.wikipedia.org/wiki/Sunlight).

Using the filmic Blender add-on I am now way overexposed. 
Here comes my first question: where do I compensate for that? I found two exposure sliders within Blender. One directly in the color management tab, which I assume doesn't affect the final rendering, but only the viewport lut. Or do I use the exposure slider under the render tab/film?

Second: How does cycles handle the light contribution from environment maps? In what relation do they stand to the sun intensity as they have a seperate intensity slider. Is there a rule of thumb to get them in the right ballpark?
Use the false colour look in combination with a CDL node (What is the the ASC-CDL node?) to bring your values within the range of the color transform. 

See also this related post: How to scale properly HDRI image, to be used in ASC-CDL HDR render?

and this one (somehow outadted, but should give you an idea): Using the False Color look in combination with the CDL Node to work on Wide Dynamic Range scenes