Hi Mike - Go to the back of the 318. On the left side of the tractor, you'll see a large threaded rod with a cross pin. That's the stop adjustment for the mower deck. With the deck raised, you can loosen (lefty loosy, righty tighty) the threaded rod. Sometimes you may have to slightly lower the deck to take tension off, but not always. Just make sure you don't loosen it so far that it starts coming apart.
Then with the gage wheels set so when you have the deck lowered to the cutting height you want, lower the deck so the gage wheels are about 1/2" above the surface. Then go back to the rear of the tractor and tighten the threaded rod until it stops.
Now, each time you lower the deck, it will stop at that same position. Hope that helps explain it. - Dave