Field fresnel placement

does anybody knows how much of the light i will lost when i place field fresnel about 320 mm from the LCD ? i have calculated custom duplet from eyeglass for about 4500 mm throw. I have LL 317mm fl fresnel.And i need it titl about 10 deg. in both vertical and horizontal .I read something about it somewhere ,but i cant find it again
 
thanks guy ,

i tested move the field fresnel about 300 mm from the frst one without lcd and i didnt see lost of light by eye.I would place lcd with polarizer to the first fresnel side .I dont know what the polarizer on the lcd will do with rays if they come parallel from the first fresnel ,reach polarizer on lcd and will continue after lcd parallel or not.If they wouldnt there is a problem of huge lost of the light .I cant test it now,cause i waiting for ffc extension .