Simple for some maybe. But speaking as a noob, the more clues the better!
I've run 3 batches through my system so far, taken fairly good notes, and should be able to make some sense of it sometime, hopefully.
Reelale - just break it down into bits:
Say you had a 10lb grist. At an average of 36 points/pound, you've got 360 total gravity points to work with, assuming complete conversion.
For a 5 gal batch, and assuming you boil off 1 gal/hr and loose 0.1 gal/lb to absorption, your total water in the system should be 7 gal (5 + 1 for boil-off + 1 for absorption loss).
360 / 7 = 51 or a pre-boil gravity of 1.051
So, with no-sparge, 1 gal of 1.051 wort will be "lost" as it's locked up, absorbed in the grain. So subtract those locked up 51 points from the original 360, convert to a percentage and you've got the theoretical max efficiency.
(360 - 1 * 51) / 360 = 86%
and
(360 - 1 * 51) / 5 = 62 or a post-boil gravity of 1.062
From here you can play with the various numbers and see how the system reacts. If absorption were .2 gal/lb, max efficiency would be 75%. Do a 120 minute boil and max increases to 88%. A monster 20lb barleywine grist would be 75%.