RE: adjusting bow to specs
I am not 100% sure about the Darton bows but the ATA that many manufacturers list on their "marketing" web site is an average for the various draw length combinations they offer. Hoyt for example may list a bow as being 36" ATA but if you look at their tune charts the actual ATA for a particular draw length or cam size may vary by up to 1/4".
As for the letoff issue, a hybrid cam (I assume that is what is on your Darton, a CPS or variant of that), can feel as if the letoff is not what it should be if the cams are out of time or under rotated. You really need a draw board to check this or have your local pro shop check the cam timing for you.