oki guys im making a lilliput projector and got the folowing thing

Fresnel Focale : 210/310mm
Varifocale : 310x360mm
lilliput
400W hqi lamp
Back face mirror
thick glas

and i have made a box and i cant figura out the focus on the fresnel when i messure the focu its as it sude be. but when i put it in the box a get a good focus when the fresnel is about 317 mm away from the lens and a haven mix up the fresnel.but here the thin when i turn around the vari focus lens then the focus is a little closer to were it sude be. am swery confused or am i missing somthin`g?. am tryng on my own to figure it out but if someone that have made a simule projector pleas contact me we can chatt on msn just ad me mr_nobody73@hotmail.com
regards
marcus