I had this same problem recently when I was about to purchase my first TT. It was rated at 7,700# gross. I had narrowed down my selection to the equal-i-zer with 4 point sway control. I used their calculator using my "best guess". Tongue weight was known at 750# and I estimated 200# in truck bed. It also recommended to 1,200/12,000 system. To say that I was skeptical would be an understatement. No matter how I played with the numbers, as long as I stayed within a realistic range it kept selecting the 1,200/12,000.
I set it up per the instructions from equal-i-zer and the setup is almost ideal.