Monitoring projector light output is a common thing to want to do. This can help you to determine when your lamp should be replaced and what your actual light output from your projector is.
The first thing you will need is a light meter. I recommend the Extech 403125(discontinued) or the AEMC CA813 if you cannot obtain the Extech. The reasons are the Extech meter actually meets the accuracy specification of about 5%. It is relatively low cost. The cosine error is less than units that use spherical correctors. When using a spherical corrector you must be very careful to be sure the corrector is not shadowed when measuring the projector.
Once you have the light meter you need to measure the light at the screen by placing the sensor near the surface of the screen with the lights off and a full white signal coming from the projector. The best way to do this is to use a test DVD or Blu-Ray that has this test pattern. The light meter should be set to measure fc (foot candles) with a 0-20 range or 0-200 range if it is above 20.
Take this number and plug it in the AccuPel screen setup calculator which can be downloaded on the left column. Using this enter the fc and screen size illuminated and you will get the projector output in lumens. You can then calculate the light level you are seeing using the screen gain. If I have tested it you can use the gain that I have measured for your screen material in this test report.