Regardless of the type of lift, he's the proper procedure for setting the deck height:
Raise the deck.
Turn the knob to the height you want.
Lower the deck until it hits the stops.
At this point, even if you lose all hydraulic fluid, the deck shouldn't move.
A few questions about your deck height knob:
Is your knob loose if you grab it and wiggle it? If not, it's probably not properly secured to the floorboards. Get underneath and look up and tighten the bolts that hold the adjustment knob frame to the floorboards.
Is your knob shaft perpendicular to the floorboards? if not, see above.
When the deck is fully raised, is your knob able to turn 360 degrees? If not, your deck isn't coming all the way up. You'll probably need to tighten the turnbuckles and/or the front yoke.