I had a lot of ups and downs this summer building our first AGP. One thing that was well worth the money I spent was a water level. Some prefer a laser, but that wasn't in my budget and things were drawn out too long to rent one. I could have bougth 3 of them for what the rental store wanted for as long as I needed it! You can make your own homemade water level, but for $40 or so you can get this fancy one online. Very accurate and easy to use, don't know what I would have done without it, highly recommended. You can check it out at: http://www.watrlevel.com.
I am in no way affiliated with the company, but it's a really slick idea. Just thought I would throw it out there for folks to check out. We are now happily swimming in our crystal clear TFP!