Various lamps produce different outputs in either lux, lumens or ft. candles depending on their design and materials. 60watts of power to illuminate a flourescent yields differently than halogen or tungsten lamps. You need to measure the relative brightness, so ....
Borrow a photographer's light meter and measure the foot candles when the lamp has full power applied to it. Note the distance in inches, feet, or whetever that you held the meter to the lamp since this is your "constant" value. Now calculate what 70% of that "full brilliance" reading would be - for example:
100 lumens at full power applied = 100% of lamp's rated output
70 lumens reading on the meter = 70% of lamp's rated output
Make sure you take measurements at the same distance to the lamp(s) since you'll have light fall-off over distance which will err your readings. The principle is the same regardless if you are measuring in lumens or ft. candles. When the meter indicates 70% of the measured full output of the lamp, you can then measure current draw and such to see how it compares to that at 100% operation.
If a photographer's calibrated meter isn't available, you can whip up a circuit quickly using a photocell, a meter and a few other support components.