Hi,
The interlacing distance is basically the distance you plan to view your print at, or alternatively the distance you measured the optical pitch.
If for example you want to print a postcard size lenticular and you plan to look at that card at a distance of e.g. 12", i.e. you measure the pitch at 12" distance, this is your interlacing distance.
Pay attention that in Lentigram, the interlacing distance is used purely for simulation purposes, together with all values in the "Simulation" box.
The value which is important for the print is the Optical pitch.
I hope this answer your question,
Giovanni