Good values for the seeds lie between +-2.0. Experiment!
Starting with res=20 makes it easy to determine the rough shape of the fractal. Refine with successively smaller values.
Limit determines how far you wish to pursue the calculations for complex portions of the fractal. If the resolution is fairly poor (say, 8) a limit of 8 or 16 is fine and can speed up drawing time. If you have resolutions of 1 or 2 a limit of 32 provides more detail (as much as you can get without color).
Set lock to 1 to ignore the mouse. Best leave this zero.