[Radiance-general] Illuminance values

[Marija]

Hi,
I have simple question: How could I determine illuminance distribution on a 
workplane?

My scene is simple:
1.Rectangular room, with ideally diffuse surfaces. Reflectances are 0.8 0.8 
0.8.
2.Room dimensions are 10x10x10 and the center is positioned in the (0,0,0).
3.In the middle of the room is placed a luminaire, with indirect light 
distribution. It is oriented toward ceiling, so workplane illuminance has 
only indirect component.

I found somewhere in the mail archives a command for illuminance calculation 
for a grid of points. For my testing room the command is:
cnt 6 6| rcalc -e '$1=$1*2-5; $2=$2*2-5; $3=-4; $4=0; $5=0; $6=1'| 
rtrace -h -oov -I -ab 2 room.oct| rcalc -e '$1=$1; $2=$2; $3=$3; 
$4=47.4*$4+120*$5+11.6*$6'>illuminance.txt

Which value is the most appropriate for -ab parameter? Should I also use 
some other ambient parameters?
I've tried with different -ab values and results are ascending, so I don't 
know which result is the most accurate.

Purpose of this calculations is to compare values from Radiance with values 
obtained form "Infinite Plane-Exitance Difference Method" approved from 
IESNA for calculation of indirect illuminance.

Thanks,
Marija