If the FL calulator tells me i need a FL of 600mm, in order to get the screen size i want from the throw length i entered, then what does that mean?

Do i need a triplet with FL =600
Do I need a front fresnel with FL = 600
Both?

Are the only fresnels available here for the 15" LCD as im going to make a 17" vertical and ive started thinking about parts.

I was planning on a zoom lens so i could put the PJ at the back of the room, but they seem a long time in the waiting, so if i can do it without then that would be cool.