The 10% rule is a good one. In teh case of most of our cats that is about 100-150 lbs of tounge wight. I know that class 1 hitches are rated for 200 lbs max tounge weight. Here is why balancing a trailer is so important:

1) If the weight is too far forward you will have too much tounge weight (obviously). This will cause the back end of your vehicle to lift the weight off the front wheels resutling in less ability to steer effectively on slippery road surfaces (or clean one depending on how bad it is). This is mostly dependant on your tow vehicle.

2) If the weight is too far aft (too little tounge weight) then the vehicle is "holding" the trailer down and the trailer wants to whip (like in Mary's case) which will toss the back end of your vehicle around and wear out the trailers suspension and bearings as well as your vehicles.

So, you want to lod the trailer such that the tounge weight does not depress the rear suspension of your tow vehicle too much, but just enough. I know this is not exact, but you can feel the difference between a properly balanced trailer and on that is not. Very tow vehicle dependant.

For example. On my VW Passat 1.8T (yes a 150 HP 4 cyl), manual. I tow the TheMightyHobie18 on a Trailex trailer. I line up the front cross bars of the boat and trailer. I usually have the mast top out as far as the rudder tips are when locked up. This gives me about 125 lbs of tounge weight and great balance. The car steers very well in all conditions.