designed box for 7" vertical but it will only focus at 330mm from mirror not the lcd panel.

this is the setup.

arc of lamp to fresnels 220mm unsplit.
20mm gap to 7" lcd.
lcd to mirror at 45 degree angle measures 130mm.
i thought that would leave 200 mm to focus on rear of triplet .
i have 50mm travel on focus box, but still short around 100mm.
using ordinary mirror, couldn,t find fs in uk.
thanks for any answers
don,t want to extend box if i can help it.

The focal length of the triplet (320mm ?) is the closest you would ever want the lens to be to the LCD. That is for a screen infinitely far from the projector. As your screen gets closer the triplet moves further away from your LCD. You can use the focalcalc 2 program in the build tools section, plug in your numbers and it will tell you the approximate distance from lens to LCD. Also, I think this is usually measured to the middle of the lens, not the back of the lens.

You do have it right though. The distance should be the total of LCD to mirror and mirror to lens.

David.