You can use the ColorRamp to visually modify the attenuation across space.
What the setup does is "normalize" the ray length to make the max length you want to control equal to 1.
In the following image, the plane is exactly 5x5, so the falloff should be 2.5 units in order for the light to go off exactly before it reaches the edge.
The ColorRamp factor input takes values between 0 and 1 while the ray length is in absolute blender units (length). So in order to use the ColorRamp we need to bring down the maximum ray length we want to control to correspond to 1. So, let's say we want to shine light up to 10 meters (or units), we'd need to divide Ray Length by 10, so that the end of the ColorRamp corresponds to 10 meters from the light source.
The ColorRamp then assigns a color to each ray depending on their length. This result (which is a color - different for each ray) is plugged into the Emission color node so that we can pass color information in addition to strength.
We still need to make the light falloff constant so that the default attenuation does not interfere (multiply) with the our gradient. So we plug in the
Constant output of a Light Falloff node into the Emission
In the final setup, we control the distance by modifying the division value (
2.500 in the screenshot above) and the strength by tweaking the Strength input (
100.000) of the Light Falloff node.
Using the additional info from cegaton's answer and Booth's edited question, I have edited this answer to be correct and complete. If the reader wants to see how we got here, visit the history