Would some of you guys talk a little about reading the jib slot?

Earlier this summer I took off the 4-way jib system from my P18-2. We never seemed to use the adjustment it provides and the cable was a pain (literally) for my crew. By removing this, I effectively moved the jib car out a couple of inches and as a result seem to have lost some power in heavier air.

In lighter air (less than 12 mph or so) I'm generally able to make the tales flow well. I was out yesterday in some nice steady 15-18mph air and almost every time the boat got moving nicely the upper lee tales would turn toward the front of the boat! It just feels like I just don't quite have the power for the wind speed!

So, assuming the slot is open too wide, when I close it back up, how do I know when it's closed too much? Same measure... power feel?

As always, thanks for the insight.