If you know the lumen output of your ligth you can recalculate it to lux, using the height from lamp to bottom and area of the tank, if the lamp is on top of the tank, you can use the volume of the tank:

Lux = 1000 x lumen/ vol (L)

I use these standards to compare the lighting:

Low - 10000-20000lux
Medium Low - 20000-40000lux
Medium - 40000-60000lux
Medium High - 60000-80000lux
High - 80000-100000lux

Somewhere i read that direct sunlight on a clear day was rated to 100000lux, and i used the low 10000lux as given from a standard 15W floresent (apr. 500lm) in a 54L tank.

Sometimes i use a wider medium light scale:

low 0-15000lux

medium low: 15000-30000lux

medium 30000-60000lux

medium high: 60-80000lux

high: 80- 100000lux