Hi all -
I build a standard lumenlab box design for a 7" lilli, but the box is larger then the LCD and therefore a lot of light is being wasted as it is being focused around the lCD.

What I would like to do is use the first fresnel (between LCD panel and light source) to focus as much light as possible onto only the LCD, rather then to spread the light equally across the whole interior of the lcd (Which is what happens when it is placed at its focal length as described in the LL manual).

How do I go about calculating the distance to do this?

I have the standard lumenlab lens kit -

you really can't!
The only distance it will focus at is at the focul length.
Yes ! you are wasting light but you are also getting very even light because of the small included angle.
To correct you would need to get a smaller lens that will focus much closer and still cover the 7" screen.
If you just move the existing lens about 1" to 3/4" closer it will help the total output and still be OK because you don't need to reach the far edges of a 15" screen.

Kirk