Steve wrote:

I turn it on and get a horizontal line on the wall. The line is obviously
not straight though.

Perhaps you mean straight but not level?

...the device doesn't have a leveling bulb in the horizontal position.
In this position only verticle leveling is allowed.

Can you rotate the linemaker grating 90 degrees?

I just want to know how to project a level horizontal line on the wall.

You might measure to make each end of the line the same distance down
from the ceiling, or use a water level, eg 10' of 1/2" clear tubing...

Nick