Question on light from Venus and full moon

I have a Q for someone here.
What is the formula for derriving flux (as a measure of light per unit
area per unit time) from the astronomy magnitude scale, for Venus
{-4.40 max} or the full moon {-12.7)? I saw this chart:

http://www.lakeheadu.ca/~physwww/cou...b/apmagdef.gif

But it don't have the range for Venus or the Moon. I know the bright
star Alpha Carina (Canopus) has magnitude of - 0.72 which is of flux of
something like 6.8 x 10-5 W/m2. (6.8 x 10^-5 watt per squre mtre).
Thnx
Robert