The bulb is supposed to sit at the focal point of the reflector, that is where the max amount of light will be reflected out to the tank.
The exact point depends on the shape of the reflector. But most should have clips or other mounting hardware that position it correctly on the endcaps.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.