Next time try dropping the whole FEL and BH to mow. With practice you can drop both in under 5 minutes. A 16" chunk of 8x8 or railroad tie under the BH makes it much easier. You'll be surprised how nimble the 2305 feels and how nicely it treats your lawn with the weight gone.
You don't need to remove the floorboards to muck with the knob. Remove the deck instead; it's easier. But for starters, remove neither.
Follow these steps:
Raise the deck fully.
Turn the knob to lock position.
Lower the deck.
Look at the gap between the lift arm and the stop *with the deck on.* It's between arm A and stop B in this picture. If your deck is drifting down this is probably a big gap.
Raise the deck.
Turn the knob to install position.
Lower the deck to the ground.
Disconnect linkage "C." Tigheten it until you think the gap between "A" and "B" will be about 2mm when the knob is in the lock position.
Lather, rinse, repeat until it's no more than 2mm on both sides.
The manual tells you to make this adjustment with the deck off; this will result in your deck being too low. You need to look at the gap with the deck weight on the arms. You may also need to tighten or loosen the front yoke as you do this.