Keep it simple.

A lens formed image like one produced by an enlarger or camera will have Cosine to the 4th falloff.

An easy technique that is free of the optics problems is to use low wattage incandescent lamp i.e. 7.5 watt, four or more feet from the film. A dimer is useful to lower the output. Control the time with an electric timer.

A density of 1 to 1.2 will make it easy to see the differences.

Expose to a mid-scale density.