If you want to properly projection map, you need to measure the amount of lumens per square meter. As a rule of thumb, anything less that 100 lux will look washed out (the ideal range is over 250 but it is usually too expensive). 500 lux is the classical amount of light to be able to work in office conditions.
To calculate your projection area multiply your projector distance by the lens multiplier (0.8, 1.0 etc) and you will get the projection width. The height is derived form the proportion of your projector (16:9, 4:3, etc). Once you have the area, use this formula
lux = lumens / (square meters)
Take into consideration that this formula is modified by lot of factors, including:
- Surface reflectiveness (can be increased or decreased depending on the material)
- External light conditions (not only the building lights but also the public lights will affect a lot)
- Atmospheric conditions (extreme cold, dusty environments, etc)
I hope this helps.
