Hey Dave,
I have a Howatt Hunter and the best tuned brace height for me and that bow is 7.75 inches. I measure this with a ruler from the deepest part of the grip to the string. I am not sure how the grips on the Mamba, Hi Speed and the Hunter differ but that should get you statrted.
When you shoot the bows and fine tune the set-ups more precisely, be sure and measure (and record some where you can find it) the final brace heights. I check mine fairly often just to make sure everything is staying the same. A new string will stretch of course, and even a well broke-in string will move in wierd ways with weather and such.
Hope this helps.
OkKeith