when i made mine i did the same thing. i figured out where i wanted it to start and stop on my waist and measured that for the top. then figured out how long i wanted it to go on my leg. measured that.
i made the bottom a couple inches wider than the top to get the trapezoid shape.
i havent split mine in the back, but i might before im done.
this was the only photo i could find right off with a close up of the kama
Take a sheet or towel, and pin to the shape you want. That way you will see how it fits and looks. Then transfer to fabric you are going to use. Daughter and I bought a cheap bed sheet set to pin and cut into templates. Hopes this helps