I know these guys are trying to make a living selling this stuff but it seems like a huge markup for what you're getting... (it's just a pile of mud in the end, afterall)...
Well I am one of these guys
That said I am on the forum to help.
For the placement, you want to do 40% of the weight distributed toward the back of the trailer and 60% towards the front. At the end the tongue weight should be right around 10% of your trailer. And that is not easy to do as I just built one.
For the height of the dome, you just opened a big can of worm
Many people believe in a 63% door to dome ratio. I personally tend to disagree and have a few people testing it out for me.
But you have the right idea, make a circle, build a sphere shaped pile of sand and place your bricks around it.
Can't wait for other members to chime in, that is going to be a fun one.
Let me know what else you need.