Yeah, I found the best way to tweak the levels is to start, as it sounds you did, with the control rods all the way up, mine extend into the overflow tube approximately half an inch.. I actually snipped the rods off so they don't stick up 8" off the top of the tank!
I also found that once the level in the tank is set and you don't get gurgling, let it run for an hour or so, and then remove water from the sump as necessary to get it to the appropriate level.
Water changes always bring about the need to "recalibrate" to a certain extent, I don't have to touch the control rods though.. what I'll do for the water change is turn everything off, this of course causes water to backflow into the sump. I'll remove 15% of the water and then start filling it back up.. I fill until the moment water starts flowing back down the tubes and then I stop adding water, and turn the pumps back on..
This inevitably leaves me with too much water in the sump after it's run for half an hour or so, so I then remove a gallon or two from the sump and everything seems to stay in balance from that point on.
If you've been following a similar approach and too much water is still filling into the sump, then it sounds like your pump isn't the right size for the GPH
that the overflows are capable of handling. I have the 125g setup, that actually uses two overflows and two returns, and I have a 1200GPH pump splitting the return to both tubes, and it seems to work OK.
I had originally tried to set it up the same way, only with a single 600GPH pump and I had problems similar to what you describe.
hope this helps!
Also - Marineland technical support online is usually pretty responsive to email questions regarding getting it setup properly.