The new meassurement should be in acks and you can put one ack oer meter, one ack or per feet, or per mile, kilometer, lightyear, whatever you prefer. From this the grit should be generated, and in the grit settings you can say, 4 lines per ack or whatever.

Of course any name other than ack would be better, but just as an example.

*EDIT*: would even be better when the grit is automaticly generated, when zooming in it skips to centimeters, milimeters, micrometers... or inches when you use the english measurement system.