Flemming,

First of all, there were two different grid map systems in WWI, as a completely new system was introduced in 1918.

For the earlier grid system, the best way to make sense of it so find one of the maps used in a KTB. Note that maps were four color, so when reduced to black-and-white they can be a bit difficult to read.

I'll send you the overview map that I found in a KTB.

Best wishes,
Michael