Alternatively, start calibre from a command prompt. I have several different configurations that I use, so for me changing the system environment variable doesn't solve my problem.
Example for one of my test configurations:
Code:
d:\CBH_Data\calibre.git>type test_calibre_small.bat
set CALIBRE_CONFIG_DIRECTORY=D:\CBH_Data\calibre.git\config_test_small
calibre-debug -g
d:\CBH_Data\calibre.git>