I recently purchased a 240 as well, and reading the manual a number of times to figure out what they were talking about, it sounds like the correct amount of mast rake is about 6" to aft measured at the gooseneck. To measure this, just attach something heavy (wrench, plumb bob, etc.) to the main halyard and let it dangle just above the boom. If it is centered on the mast and 6" aft of it, then things are in the right spot. If not, then you will need to adjust the tension on the shrouds to get it there.
Here is the "official" text from the manual:
7. After the rig is centered, set the amount of mast rake to approximately one degree of aft angle. This can be measured by hanging a weight, such as an adjustable wrench from the main halyard shackle and adjusting the halyard so the wrench is suspended immediately above the boom. With the boat level, this wrench when hanging from the main halyard above the gooseneck should be 6
” (15cm) from the aft face of the mast. Adjust the forestay turnbuckle as necessary to achieve this position.