IGT volume reconstruction: Output spacing

I am using the Volume reconstruction feature from the IGT module and have a question about the Output spacing parameter specifically.
I am using ultrasound data obtained from manual sweeps, about in the same direction as one of the axes (K) and the other dimensions (I, J) define the 2D images.

Are there instructions somewhere about a systematic method to adjust these values? At the moment I do it by trial and error, by using the original scaling factor for I and J, and by decreasing K incrementally.