In theory, the fL is a mathematical relationship of your projector output and the screen size assuming a gain of 1 at the screen. There are many
factors that can make the image dimmer than expected (such as dull projection surfaces, ambient light, bounced light, etc...) and very few
factors that can make your image brighter such as an expensive projection screen with a gain >1.
Your projector is 3K so in theory you will get 48 fL on a 10'x6.25 screen, 33 fL on a 12'x7.5' screen, 16 fL on a 16'x10' screen and 12 fL on a 20'x12.5'..... But, it doesn't look like you are projecting on projection screens and there is tons of ambient light so chances are you are not getting the numbers above. Probably far from it! You can make a copy of the spreadsheet I sent you and plug in your own numbers (just disregard the LCD size since that projector is DLP)
Also, your projector is 1280x800 native so anything else you put in there will not be optimal. For instance your projector can scale to 1600x1200 input but that's 4:3 ratio whereas your projector is 16:10 so in order to display 1600x1200, the image would be 1067x800 so you wouldn't be projecting full width. Here is a good article on native resolution vs maximum resolution: http://www.projectorcentral.com/maximum_resolution.htm