Sky definition


I want to model a sky that gives a ground illuminance of 50,000 lux.
The sky will be clear and at 7am during winter in Auckland New Zealand
(longtitude 174.76, latitude -36.85).

What is the easiest way to define the sky in terms of the required
radiance and irradiance values? I dont have any other info apart from
the resulting ground illuminance.

We are interested on the maximum levels of illuminance on the museum
and 50,000 lux has been identified as the maximum expected.
I dont believe using the daylight factor approach would be the best
way to compute the interior illuminance so I would be calculating the
illuminance directly.

Do I just need to estimate the solar horizontal and diffuse illuminance
values? If so, what would be realistic values??

Ben Masters