how high off the ground should the ball on my hitch be?

Correct Answer: I went to Lowe's and moved a Carry-On 4x6 trailer to a level spot in the parking lot, put a level on the coupler bar and measured from the bottom of the coupler to the ground. It measured 11 3/4. I ended up buying a used Carry-On 4x6 and using the same method, it measured 11 1/2. I contribute the 1/4 inch difference to manufacturing and assembly tolerances as the coupler bar bolts to the trailer and likely has a small amount of variance. Regardless, you should be able to use these measurements to determine the proper ball mount rise or drop to use with your hitch receiver. https://www.etrailer.com/faq-ballmount.aspx

Often depends upon your tow vehicle. Perhaps this will help: http://i48.photobucket.com/albums/f249/woosterken/Scan10003.jpg The main thing is that your trailer runs level. You may have to buy a riser or a drop to accomplish this because each vehicle differs as do the trailers tongue height. Example: My S-10 Blazer sits 18" to the top of the ball and has a slight downgrade towards the back. My Sons Chevy Z-71 sits at 21" to the top of the ball and it has a drastic slope until he purchased a 3" drop hitch. If I were to hook that up to my father's minivan, I'd need to purchase a 3" rise hitch or take my son's drop and turn it and the ball over. Hope that helps

I dont think it is too critical. You want the trailer to be close to level and about 15% of the total weight should be on the tongue.