As we have all discovered, screen brightness is the biggest battery killer on the Desire HD, so correct settings are crucial.
Like the correct settings on a Toaster, there is actually only a very small amount of change needed for brightness. I find 10% for low, 15% for Normal and 25% for High is all I need
Using the slider is almost impossible to get an accurate setting.
There is a widget included with the Desire HD which toggles between 3 different brightness control, but it's settings are about 25% brightness at lowest, 50% and 100% without anyway to change them.
Install this app below and you can set these settings yourself. No more battery draining.
Brightness Profiles - Android app on AppBrain