I downloaded the program and it seems really hard for me to draw and gentle curve also.
What I do when designing a bargello is chart it out on graft paper. it doesn't have to be exact, just so you can get an idea what the design will look like. I usually star on one site and draw a rectangle to represent 2" h x 3-1/2 wide. then step down bottom of that rectangle and the next space would be 2x 3in., then 2x 2 2-1/2, 2x2, and so on until you get to and 1/2 in If you like that curve, you may want to repeat going in the opposite direction. You can repeat this all the way across the quilt.