Lpi means how many lines per inch,such as 18lpi means there are 18 lines per inch,25Lpi means there are 25 lines per inch.But for file interlacing,we need to know the accurate number such as 18.29 or 25.62.

First,print the calibration chart by your printer.
Second,moving the lentiuclar around over the chart and alight the lines straight,there will be white color alternated with black color  from different angle.
Third,find which one can totally show white color or black color from a fixed angle.it is the right pitch for file interlacing.