Tongue weight formula???

I am planning on building a grasshopper trailer (4x8). I am using a Geo Tracker to lug it around. I have 150lbs tongue weight. How do I figure out my tongue wieght? Is their a formula? or a method of determining? Do I figure this all out after the trailer is loaded? Any advice will be greatly appreciated. If you could point me towards some grasshopper designs would be appreciated. 
